Le Plaisir aux noix is a creation of Gaston Lenôtre, the founder of "Lenôtre" a culinary empire. The original plaisir aux noix filling is based on crème Anglaise buttercream lightened with some Italian meringue. It can also be achieved using mousseline cream. This version is based on creme diplomate; easier to execute, lower in fat and sugar.
Toss walnuts in sugar and cook until caramel forms; stirring constantly. Let cool. In a running food processor, blend caramelized walnuts to fine coarse and save 2.7 oz/80g for the succès biscuit. Continue to blend the remaining mixture to paste (praliné) for the diplomate cream. Set aside.
In a food processor, process almond meal, walnut meal, and powdered sugar to combine; add milk. For the meringue, beat egg whites with cream of tartar and one-third of the fine white sugar to stiff peaks – add remaining fine white sugar at it goes. Fold one-third of the meringue into the almond-sugar mixture. Add remaining meringue and the previously saved caramelized walnut meal. Turn oven on. On a baking tray lined with a silicone mat or parchment paper, pipe out two 8''/20cm diameter disk using the #808 pastry tip (one penny /16mmØ). Make 2 extra smaller disks with remaining mixture if so.
Bake succès biscuit at 260ºF/130ºC for 1 hour and 10 min. Turn oven off and leave it to dry out for 20 to 30 minutes more. It should be firm to the touch.
Heat up milk and vanilla. Meanwhile, beat yolks, sugar and starch. When milk is boiling, turn the heat off – temper yolk-mixture with one-third of the hot milk. Add yolk-mixture into remaining hot milk – bring to boil and cook for 2 minutes; whisking constantly. Turn off heat, mix in the softened gelatin and mascarpone. Transfer onto baking tray lined with plastic wrap; film in contact – cool and chill completely. Beat chilled custard on high speed, add walnut praliné, port and the whipped cream. Set aside.
Place a succès biscuit (flat side down), pipe out 12 large dollops of walnut diplomate cream from the edge of the succès and fill up the center. Coat the surface of the second succès biscuit (flat side up) with a thin layer of filling and top with toasted sliced almonds. Place it on top of the cake like a sandwich and refrigerate it for 6 hours before eating (best overnight). Dust with powdered sugar or snow sugar (it wont melt). Walnut dream cake can be refrigerated for up to 4 days or kept frozen for up to 3 months. Bon appétit!