For the ganache:

1. Place the dark chocolate in a medium heatproof bowl. In a small saucepan, bring the cream and inverted sugar to a boil over medium heat.
2. Remove from the heat and pour over the dark chocolate. Let stand for 2 minutes to melt, then stir with a silicone spatula to combine.
3. Add the hazelnut paste to the chocolate mixture and stir to combine. Add the butter and stir until melted. Set aside.

For the base:

1. Put the feuilletine in a medium bowl and crush it further by hand. In the top of a double boiler (or a heatproof bowl set over a pot of simmering water), combine the milk chocolate, hazelnut paste, and cocoa butter. Cook, stirring over medium heat, until melted and combined.
2. Remove from the heat and stir the mixture until a candy thermometer registers 100 degrees, 8 to 10 minutes. Stir in the feuilletine.
3. Line a 9 by 13-inch baking sheet with plastic wrap, leaving some overhanging the sides for easy unmolding. Spread the base into the pan using an offset spatula to create a smooth, even layer. Pour the ganache on top and spread it in an even layer. Cover and let the cake set at cool room temperature for 8 hours.
4. To serve, lift the cake out of the pan and transfer to a cutting board. Fill a tall pitcher with very hot water. Warm a long, sharp knife by dipping the blade into the water, then wiping it dry. Use the knife to cut the cake into 24 equal rectangular pieces (each about 4 by 1 inch), warming the knife in the water and then wiping it off between each cut.
5. Before serving, use a pastry brush to brush the cocoa powder on top and around the sides of the each piece. Decorate the top of each with a stencil using the edible gold powder.