Step 1

For the crust, line a 9 x 13 inch pan with parchment paper, allowing the parchment to hang over the long sides of the pan to act as handles.

Step 2

In a large bowl, combine melted chocolate and chocolate-hazelnut spread and stir until smooth. Fold in the feuilletine until combined. Transfer the mixture to the prepared pan. Use a sheet of wax paper (or a nonstick Silpat mat) to press the mixture into a smooth, even layer. Transfer to the refrigerator to chill while making the filling.

Step 3

For the ganache, place the chopped chocolate in a heatproof bowl. Bring the cream to a simmer in a small saucepan over medium heat; pour over the chocolate. Let mixture sit for 2 minutes, or long enough to melt the chocolate. Add the butter and whisk until ganache is very smooth. (You can also use an emulsion blender to mix.)

Step 4

Pour the ganache over the crust and smooth with an offset or rubber spatula. Transfer to the refrigerator until set and firm, about 1 - 2 hours. Slice into bars before serving. Keep stored in the refrigerator and serve chilled.