This recipe is based on a post by the excellent La Pâte de Dom which he calls 'Cracahuète', being a bit of word play between croquant(crunchy) and cacahuète(peanut). Not being fond of peanuts in cooking, I have switched the nuts to hazelnuts of which there are plenty on the trees. The recipe serves 10 to 12 since it is very rich.
Like much modern pastry cookery, the tart filling is a sort of 'club sandwich' of several ingredients. These would be on hand in a professional pastry kitchen and therefore the tart is little more than an assembly job. In the domestic kitchen making each element from scratch for a single tart is possibly more trouble than it is worth: frankly one can use as many layers as one likes.
The basics elements are a sweet pastry case into which are layered in turn, salted butter caramel, chocolate custard, chocolate and hazelnut crunch, and hazelnut-flavoured white chocolate ganache. The tart is iced with shiny chocolate icing.
As this is the second time of making the tart, I already had some frozen sweet pastry and a pot of salted butter caramel in the fridge, so the things needed were custard, ganache and hazelnut crunch.
The recipes for the main ingredients are below. The main technical issue is that since three of the fillings are soft, these need to be frozen or very cold during assembly or the result will be just a mess. This means that it is easier make the tart over several days, leaving time for each ingredient to become very cold.
The assembly comprises:
i. Blind bake a 20 cm round tart shell in the standard way.
ii. Spread salted butter caramel on this base. Freeze until the caramel is hard.
iii. Spread or pipe on the very cold chocolate custard. You can easily level the surface by pressing a cling film wrapped disc of the correct diameter onto the surface. Refrigerate until firm or for up to 24 hours..
iv. Ice the frozen ganache disc. Place the frozen crunchy layer on the custard and top with the still frozen iced ganache. Hold in a cool spot not more than 15°C colder than your room temperature. If it is colder than that, condensation will form on the icing and spoil the finish. So if you need to refrigerate the cake, put it a closed box. Take the box out of the refrigerator an hour or so before it is to be eaten to allow it to warm up slowly. Decorate as you like.
Salted Butter Caramel
Vanilla powder 2grams
Glucose Syrup 50grams
Glucose Syrup 100grams
Salted butter 70grams
Bring the liquids and 50 grams of glucose to a boil, caramelise the remaining glucose and sugar, pour on the nearly boiling liquid and stir well to dissolve. Stir in the room temperature butter and salt while the mix is still slightly warm so the butter blends but does not melt. Store refrigerated in a jam jar. These quantities are enough for two tarts. The mixture will need warming a little in the microwave to soften it enough to spread or pipe.
Dark Chocolate Custard
Glucose syrup 20grams
Egg yolk 26grams
Dark Chocolate 112grams
Mix together all the ingredients except the chocolate, heat gently to make a custard. The mixture shouldn't boil so a double boiler or a Simmertopf are handy. Let the mixture cool a bit and chop the chocolate into bits and stir in to dissolve. You can sieve it to be sure there are no lumps and store in the refrigerator for a day or two with the surfaced covered with plastic wrap so a skin cannot form.
Milk Chocolate and Hazelnut Crunch
Milk Chocolate 15garms
Praline Paste 60grams
Toasted hazelnuts 10grams
Ice cream wafers 40grams
Praline paste and crunchy ice cream wafers are available commercially. The latter are sometimes called Crêpes dentelles or Wafer rolls. There are recipes for hazelnut praline paste on the web if you can't buy it.
Melt the chocolate and praline paste in the microwave until liquid and stir in the lightly broken ice cream wafers and the coarsely chopped toasted nuts. Spread into a thin18cm disc on a cake base or plastic container lid. Wrap in cling film and freeze until needed.
White Chocolate and Hazelnut Ganache
Gelatine powder 3grams
Toasted hazelnuts 40grams
White chocolate 50grams
Praline Paste 80grams
Heat the milk and infuse with chopped hazelnuts for at least an hour. Soak the gelatine in water for 20 minutes or more. Strain the milk and reheat it, if necessary, to about 60°C and add the gelatine mix. Gelatine doesn't like to be heated too much because it will lose its gelling power. Melt the chocolate in the cream with the praline paste in a microwave, stirring from time to time, until dissolved and mix into the gelatine milk. Mix well, sieve if you like, and store over night in the refrigerator with the surface covered in cling film to prevent a skin forming. A day or two later whip the mixture until light and pipe or spread into a suitable 18 cm circular mould. I used the bottom half of this rather expensive one. https://www.meilleurduchef.com/en/shop/pastry/cake-mould/shaped-moulds/sil-silicone-mould-kit-eclipse-silikomart.html
It gives an attractive dome shape, but the world wont end if it is a flat disc. It just has to fit inside the pastry case. Freeze the filled and wrapped mould until needed.
Shiny Chocolate Icing.
Gelatine powder 6grams
Glucose syrup 38grams
Cocoa powder 38grams
The icing is prepared ideally on the day the tart is to be served. Soak the gelatine in water for at least 20 minutes. Heat the glucose sugar and water to 103°C and stir in the sifted cocoa or chocolate powder. Stir thoroughly to produce a smooth mix. Heat the cream briefly in the microwave and add to the chocolate mixture. When the mix has cooled to 60°C stir in the gelatine mix.
The next step is to try to remove any air in the mixture so no bubbles form later. The ideal equipment for this is a small stick blender. In a tall narrow container blend the mixture for 5-10 minutes with the blender head always under the surface. After a while the bubbles on the surface will disappear and the mixture will look shiny. Sieve the mixture into a plastic jug and cover with cling film. At this point the mixture will still be quite warm. It needs to cool to 34°C so it is thick enough to pour as icing onto the frozen ganache. When the mixture is at 34°C, place the frozen ganache on a support on a plate, a 10cm pastry ring is ideal, and gently pour the icing onto the ganache until it is entirely coated. The excess will run off onto the plate. The icing should be completely flawless like glass.
Finish the assembly by placing the frozen crunchy disc onto the custard in the tart shell, and gently lowering the iced ganache in place. Two forks are best for the job, with the tines under each side one can gently lift the ganache off its support and lower it onto the filled tart shell. Or, less successfully, onto the floor and into the dog.
The Pâte de Dom 'You Tube' video shows each step in exact detail.