The recipe is a little involved - you have to chop chocolate, and toast nuts, and make a caramel for the praline paste, and dirty more than a few dishes. This recipe is worth it though, I promise.
Regarding the praline paste needed for this recipe: I ran into a snag while making the praline paste from the book (although you'd never know it in the final product), so below I linked to a similar recipe with a different technique that I think will work better. Let me say that the praline paste is so delicious all by itself that I have been slowly whittling away at the leftover praline paste in the fridge. Where has this stuff been all my life?!
Chocolate Hazelnut Truffle Squares
adapted from The Good Cookie by Tish Boyle
Makes 16 bars
hazelnut brownie layer:
1 1/2 ounces bittersweet chocolate, chopped
4 tablespoons unsalted butter
1 large egg
1/2 cup granulated sugar
1 tablespoon hazelnut liqueur
1/4 cup all-purpose flour (or gluten-free flour mix- I used Cup4Cup)
1/2 cup hazelnuts, toasted, skinned and coarsely chopped
hazelnut truffle layer:
6 ounces bittersweet chocolate, chopped
6 tablespoons unsalted butter
1/4 cup praline paste (store-bought or recipe here)
1/4 cup granulated sugar
pinch of salt
1/2 cup heavy cream
1 tsp pure vanilla extract
1 large egg
2 large egg yolks
confectioner's sugar for dusting
16 hazelnuts, toasted and skinned
Preheat the oven to 350 degrees F. Place about 3/4 cup hazelnuts on a baking sheet (add 1 cup hazelnuts and 1 cup blanched almonds to the baking sheet if making the praline paste recipe) and bake in the oven until fragrant and toasted, about 10 minutes. Rub the hazelnuts inside a clean towel to remove as much of the skins as possible. Reserve 16 of the barest, nicest looking hazelnuts to use as garnish. Divide the remaining as follows: 1/2 cup hazelnuts for brownie layer and 1 cup hazelnuts and 1 cup almonds for praline paste, if making.
Make the hazelnut brownie layer:
Reduce the oven temperature to 325 degrees F. Spray a 9x9-inch pan with cooking spray (or coat with softened butter). Line the pan with parchment, allowing the excess to hang over two sides, then spray or butter the parchment. Coarsely chop the 1/2 cup reserved toasted hazelnuts. Place the chocolate and butter in the top of a double boiler over barely simmering water and heat while stirring until melted and smooth. Stir in 1/2 cup sugar and the hazelnut liqueur until blended. Add the egg and stir vigorously until well blended. Fold in the flour and hazelnuts just until the flour is incorporated. Spread the brownie base evenly into the prepared pan.
Make the truffle layer:
Place chocolate and butter in the top of a double boiler over barely simmering water and heat while stirring until melted and smooth. Remove the pan from the heat and stir in the praline paste. Whisk in the sugar, salt, heavy cream, and vanilla. Whisk in the egg and egg yolks, one at a time, blending well after each addition. Scrape the mixture over the brownie layer and spread it evenly.
Bake the bars for 25-30 minutes until the top is puffed and set and no longer shiny. Cool the bars completely on a wire rack, then chill for at least 2 hours.
Cut the bars into squares using a sharp chef's knife. For clean cuts you can run the knife under hot water, then dry the knife and make your cut, cleaning off the knife between cuts. Dust the bars with confectioner's sugar and press a toasted hazelnut into the top of each bar. Serve with vanilla ice cream.