Over a double boiler, gently melt the chocolate. Once it's melted, turn off the heat and stir in the nutella until it is smooth. Add the Feuilletine and fold it into the chocolate. Pour the warm mixture into the prepared pan. Cover it with plastic wrap and press the mixture firmly into the pan. Leave the plastic on the base and refrigerate it while you make the mousse.
To Make The Mousse
Melt the chocolate and peanut butter over a double boiler. Cool to room temperature.
Whip the heavy cream and vanilla to soft peaks. It should droop on the whip when lifted out of the bowl, but not fall off.
Fold 1/3 of the cream into the melted milk chocolate. It is okay if there are still some white streaks. Add the next 1/3 of the cream to the chocolate mixture and gently fold it together. Add the rest of the cream and fold together. Pour the mousse over the chilled base and smooth it out with an offset spatula. Refrigerate, uncovered.
To Make The Ganache
In a saucepan, heat the cream over low heat until it starts to simmer. Add the chopped chocolate and swirl the pan until the chocolate is submerged. Let it sit for 3 minutes and then gently stir it together with a rubber spatula, until smooth. Let it sit to room temperature.
Pour the room temperature ganache over the chilled mousse and shimmy the pan to cover the mousse completely and evenly. You can use an offset spatula if it isn’t spreading evenly by just tilting the pan. Cover with the chopped peanuts and refrigerate for at least 2 hours. This can be made a day or two ahead.
To cut the bars, run a hot knife around the edge of the pan. If you have a mold, just lift it off, if you used a square cake pan you will need to invert the bars. To invert them, you’ll want to very quickly wave the pan over the stove, just for a few seconds (not long enough to melt the base), cover the top with parchment and invert onto something flat, then invert it onto a cutting board.
Use a very thin knife, dipped into hot water to cut the bars into whatever size or shape you’d like.
Peanut Butter: Skippy or Jif is best here. The natural one you get at the co-op is too grainy. This is optional, just leave it out if you’d rather.