Make the brownie

1. Preheat oven to 350°F. Grease the sides of a 9-inch cake pan and line the bottom with parchment paper.
2. Add the flour, cocoa powder, baking powder and salt to a medium sized bowl and set aside.
3. Add the oil, sugar and vanilla extract to large mixing bowl and mix together until well combined.
4. Add the eggs and mix until well combined.
5. Add the dry ingredients and mix until well combined, but don’t over mix.
6. Pour the batter into the prepared pan and spread evenly.
7. Bake for 27-32 minutes, or until a toothpick comes out with a few crumbs.
8. Allow brownie to cool for 3-5 minutes in the pan, then remove to a cooling rack to cool completely.
9. When the brownie has completely cooled, set it up to be layered with the mousse layers. Put the brownie on a cardboard cake round, then use an acetate cake collar and secure it around the brownie. If you’d like, you can also build the cake in a springform pan (with removeable sides). If using a springform pan, I like to line the sides with parchment paper and tuck it down between the sides of the pan and the brownie to keep it in place. It makes it easier to remove the sides of the pan later and then just peel off the parchment paper. You can also let it stick above the sides of the pan about an in higher, in case your cake is a little taller than the sides of the pan. Set aside.

Make the peanut butter mousse

1. Add the cold water to a small bowl and sprinkle the gelatin evenly over it. You want all of the powdered gelatin to be touching water. If it isn’t, give it a stir until it’s all moistened. Let the gelatin sit and bloom/hydrate for 5 minutes.
2. Heat 1 cup of the heavy whipping cream in the microwave until it just starts to boil. Remove from microwave and set aside. Heat the gelatin for about 10 seconds, until melted, then add it to the hot cream and stir together.
3. Add the peanut butter chips to a large bowl and pour the gelatin and cream mixture over the peanut butter chips. Let sit for 2-3 minutes for the peanut butter chips to melt, then whisk until melted and smooth. Let sit for about 10 minutes, to cool to about room temperature. You don’t want it to be too cool, or the gelatin will begin to firm up, but if it’s too warm, it’ll melt your whipped cream, so keep an eye on it.
4. Whip the remaining 2 cups of heavy whipping cream and powdered sugar until stiff peaks form. Fold about 1/3 of the whipped cream into the cooled peanut butter mixture. It will begin to firm up quickly. Fold in the rest of the whipped cream. Be sure there aren’t any streaks of whipped cream remaining.
5. Spread the mousse into an even layer on top of the prepared brownie. Set in the fridge while you make the chocolate mousse.

Make the chocolate mousse

1. Add the cold water to a small bowl and sprinkle the gelatin evenly over it. You want all of the powdered gelatin to be touching water. If it isn’t, give it a stir until it’s all moistened. Let the gelatin sit and bloom/hydrate for 5 minutes.
2. Heat 3/4 cup of the heavy whipping cream in the microwave until it just starts to boil. Remove from microwave and set aside. Heat the gelatin for about 10 seconds, until melted, then add it to the hot cream and stir together.
3. Add the chocolate chips to a large bowl and pour the gelatin and cream mixture over the chocolate chips. Let sit for 2-3 minutes for the chocolate chips to melt, then whisk until melted and smooth. Let sit for about 10 minutes, to cool to about room temperature. You don’t want it to be too cool, or the gelatin will begin to firm up, but if it’s too warm, it’ll melt your whipped cream, so keep an eye on it.
4. Whip the remaining 1 1/2 cups of heavy whipping cream and powdered sugar until stiff peaks form. Fold about 1/3 of the whipped cream into the cooled chocolate mixture. It will begin to firm up as well, but not as much as the peanut butter mousse. Fold in the rest of the whipped cream. Be sure there aren’t any streaks of whipped cream remaining.
5. Spread the mousse into an even layer on top of the peanut butter mousse. Set in the fridge to chill and firm up completely, 3-4 hours.

Make the chocolate whipped cream and finish

1. Remove the cake from the fridge and remove the acetate collar, or remove it from the springform pan. Place on a serving plate or platter.
2. To make the chocolate whipped cream, add the of the ingredients to a large mixer bowl and whip on medium speed until medium to stiff peaks form. You don’t want it to get too thick too fast. It thickens quickly.
3. Frost the outside of the cake and pipe a border around the top edge, if desired. I used Ateco tip 847. Decorate the cake with some crushed/chopped peanut butter and chocolate chips, if desired.
4. Store cake in the fridge until ready to serve. I like to use an air-tight cake carrier. Best if eaten within 4-5 days.