For the Cake:

1. Preheat the oven to 350°F
2. Line the bottom of a 8″ round cake pan with parchment paper. Butter the sides.
3. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, about 3-4 minutes.
4. Add the sour cream, vanilla and Irish cream.
5. Add the egg and egg white in one at a time, mixing in between additions.
6. Sift together the flour, cocoa powder, baking powder and salt in a separate bowl.
7. Add the flour mixture to the mixer about 1/2 cup at a time, making sure it is fully combined before adding the rest.
8. Pour the batter in the prepared cake pan.
9. Bake for 30-35 minutes, until a toothpick or cake tester inserted in the center comes out clean.
10. Let cool for 5 minutes before removing to a wire rack or a cake turntable.
11. Use a cake wire or large serrated knife to slice the top of the cake, leaving a flat surface.

For the Mousse:

1. Pour the water in a small bowl and sprinkle the powdered gelatin over it in a single layer. Set aside for about 5 minutes.
2. In a small saucepan, bring the Irish cream and 1/4 cup of the heavy cream to a boil.
3. Remove from heat and add the gelatin to the mixture.
4. Stir until fully dissolved.
5. Add the chocolate chips and stir, then let sit, covered, for about 5 minutes.
6. Whisk until the chocolate melted and the mixture is smooth. Set aside to cool.
7. In the bowl of a stand mixer fitted with the whisk attachment, whip the remaining 1 1/2 cups of heavy cream, vanilla and powdered sugar on high speed until stiff peaks form.
8. Gently fold in the chocolate mixture half at a time, making sure the mousse is light and fluffy and fully combined.

For the Ganache:

1. In a small saucepan, bring the heavy cream and Irish cream to a boil.
2. Add the chocolate chips and stir, then let sit, covered, for about 5 minutes.
3. Stir until the chocolate is fully melted and combined and set aside.

To Assemble:

1. If using a cake collar, wrap it around the base of your already trimmed cake and secure it. Make sure the parchment paper has been removed from the bottom.
2. If using parchment paper, place about 24″ of plastic wrap in the bottom of the 8″ round cake pan with at least 6″ hanging off each side of the pan. These will be your handles to remove the cake later.
3. Remove the parchment paper from the bottom of the cake and place inside the pan.
4. Trim the parchment so that it is the circumference of your cake + about 2″ and at least 4″ high.
5. Wrap it around your cake so that it sits between the cake and the pan.
6. Pour the ganache on top of the cake, spreading it in an even layer.
7. Spread the mousse in an even layer on top of the ganache.
8. Chill in the fridge for at least 6 hours, enough to firm up.
9. Once firm, remove from the fridge.
10. If you used a cake collar, slowly peel it off of the sides.
11. If you used parchment paper, use the plastic wrap to remove the cake from the pan then slowly peel the parchment off of the sides.
12. If you need to fix the sides of the cake, run a spatula or butter knife under hot water and smooth the sides.

For the Whipped Icing:

1. In the bowl of a stand mixer fitted with the whisk attachment, whip together the heavy cream, cocoa powder, confectioners sugar and vanilla until stiff peaks form. If you’d like a more substantial frosting, add more confectioners sugar 1 Tbsp at a time until you reach desired consistency.
2. At this point, you can either use the icing to frost the top of the cake in an even layer, or you can place a piping bag in a drinking glass, tip at the bottom and fill with the icing.
3. Pipe round peaks, stars, or large roses. Then garnish with fruit, sprinkles, chocolate shavings, candy or whatever you like!