For Praline powder:
1) Preheat oven to 300 degrees F. Toast almonds and hazelnuts for 15 minutes until fragrant on a baking sheet lined with parchment paper. Meanwhile, in a small pot, stir together sugar, water, and corn syrup and heat over medium-high heat. Stir the mixture to combine everything but then let it sit so sugar can caramelize.
2) Let caramel cook until water cooks out and it turns a nice tan color. Again, don’t stir it at this point. Once caramel is a nice even color (probably 4-5 minutes of cooking), pour over toasted nuts.
3) Let praline cool until hard and then process in a food processor.
1) Keep oven at 300 degrees F. Roast hazelnuts for 15 minutes. Then use a paper towel to rub off shells from hazelnuts. It’s okay if you can’t get all the shells off.
2) Process hazelnuts with 1/4 cup sugar in a food processor until it is a flour consistency. Stir in flour.
3) In a stand mixer with a whisk attachment, whisk eggs whites with 1/2 cup sugar until they are firm and hold their shape.
4) Fold nut mixture into the egg whites using a wide spatula. Try to keep the batter as light as possible.
5) Transfer half of the batter to a jelly roll pan lined with parchment paper, buttered and floured. Bake cake at 300 degrees For 25-30 minutes until lightly browned. Then let cool, remove from the pan, and let cake cool completely. Repeat with second half of the batter.
NOTE: You can make the cake in advance, cool it, wrap it in plastic and store in the fridge for a day or two.
For Pastry Cream:
1) Heat milk and 1/2 cup sugar in a large pot over medium heat until steaming hot.
2) In a stand mixer with the whisk attachment, whisk yolks and 1/2 cup of sugar until very light and can hold a ribbon, about 4-5 minutes of beating. Whisk in corn starch near the end.
3) Whisk 1/2 of the hot milk mixture into the yolks, then whisk the tempered yolks back into the hot milk.
4) Return to low heat and stir constantly as mixture thickens. It should get very thick and easily coat the back of the spoon.
5) Let pastry cream cool and chill completely before using.
NOTE: You can make this in advance as well.
For Chocolate Ganache:
1) Heat cream until steaming hot. Place chocolate in a metal mixing bowl. Pour hot cream over chips and let sit for a few minutes and then stir well until perfectly smooth.
2) Let ganache cool at room temperature until it’s spreadable.
NOTE: You should be able to make this in advance and reheat slowly over a double boiler, but I had better luck making it right before I needed it.
To make the cake:
1) Cut cake sheets in half to have four even pieces. Place one on a large serving tray. It helps to line the cake with parchment paper for easier cleanup. Spread chocolate ganache on bottom layer.
2) Top with second cake. Stir 1-1 1/2 cups of praline powder into 2 cups of pastry cream. Spread cream over cake. You might not need it all.
3) Top with third cake. Stir 1 tablespoon rum and 1 teaspoon vanilla into 2 cups pastry cream. Spread that cream flavor on third cake.
4) Top with fourth cake and coat with chocolate ganache on top.
5) Optionally, frost side of cake with leftover pastry cream and press on sliced almonds.
Let cake chill for at least a few hours before slicing and serving!