1. Preheat the oven to 350°F (176°C). Line the bottom of an 8-inch (20cm) cake pan with parchment paper and grease the sides.
2. In a large mixing bowl, cream the butter and sugar together on medium speed until light in color and fluffy, 3-4 minutes.
3. Add the sour cream and vanilla extract and mix until combined.
4. Add egg and egg white in two batches and mix until incorporated. Scrape down the sides of the bowl as needed to make sure everything is combined.
5. Combine flour, cocoa powder, baking powder and salt in a medium bowl.
6. Add half of the flour mixture to the batter and mix until combined. Add Guinness to the batter and mix until combined. Scrape down the sides of the bowl as needed to ensure everything is being combined. Add the remaining flour mixture and mix until smooth.
7. Pour the batter into the prepared cake pan. Bake for 33-36 minutes, or until a toothpick inserted in the middle comes out with a few crumbs.
8. Remove cake from the oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.
9. Once it has cooled, use a large serrated knife to remove the dome off the top so that it’s flat.

TO MAKE THE MOUSSE:

10. Once the cake is cool, make the mousse. In a small bowl, sprinkle the powdered gelatin over the water, making sure the powder is mostly touching water. Let stand for about 5 minutes.
11. Place the chocolate chips in a medium glass bowl.
12. Microwave the Guinness and 1/4 cup (60ml) of heavy cream in a medium measuring cup until it comes to a boil.
13. Add the gelatin mixture to the hot heavy cream mixture and whisk until the gelatin is dissolved.
14. Pour the hot cream over the chocolate chips and let sit, covered, for about 5 minutes.
15. Whisk the chocolate mixture until the chocolate is melted and the mixture is smooth. Set aside to cool to about room temperature.
16. Whip remaining 1 1/2 cups (360ml) of heavy cream, powdered sugar and vanilla extract with a mixer fitted with the whisk attachment until stiff peaks form.
17. Gently fold about a third of the whipped cream into the cooled chocolate mixture until combined. Add the second third of the whipped cream and gently fold into the chocolate mixture. Add the remaining whipped cream and fold together until completely combined. Set the mousse aside.

TO MAKE THE GANACHE FILLING:

18. To make the ganache filling, place the chocolate chips in a small glass bowl.
19. Microwave the heavy whipping cream and Guinness in a medium measuring cup until it comes to a boil.
20. Pour the hot cream over the chocolate chips and let sit, covered, for about 5 minutes.
21. Whisk the chocolate mixture until the chocolate is melted and the mixture is smooth.

TO PUT THE CAKE TOGETHER:

22. To put the cake together, you can use one of two methods. The first would be to use a plastic cake collar. You can usually buy them online. If using this method, begin layering the cake by wrapping the collar around the cake layer and securing it in place. The other method would be to build the cake in an 8 inch (20cm) cake pan or 8 inch (20cm) springform pan. I recommend an 8×3 inch pan so it’s a little taller. Place some clear wrap in the bottom of the pan, extending up the sides (to remove the cake from the pan later). Use parchment paper to create a mock collar, then add the cake layer to the pan.
23. Spread the ganache filling into an even layer on top of the cake layer.
24. Spread the mousse into an even layer on top of the ganache.
25. To create the pattern on top of the cake, use the rounded end of a 9 inch (23cm) offset spatula. Lightly press the spatula into the top center of the mousse. Keeping the spatula lightly pressed against the mousse, use a turntable to spin the cake and slowly move the spatula to the outside of the cake, creating a spiral.
26. Place the cake into the fridge to cool and firm, about 5-6 hours.
27. Once the cake is firm, remove it from the refrigerator. If you used a plastic cake collar, carefully peel it off. If you used a cake pan with parchment paper, use the plastic wrap to lift the cake out of the pan, then peel off the parchment paper. If needed to smooth the sides of the cake, use a warm offset spatula to gently smooth out the sides.
28. To make the whipped cream, whip the heavy cream, cocoa, powdered sugar and vanilla extract at high speed with the whisk attachment until stiff peaks form.
29. Pipe several large roses along the outside edge of the cake. Pipe some additional little flowers and add some sprinkles, if desired.

Store the cake (in an airtight container, if possible) in the refrigerator until ready to serve. Cake is best for 2-3 days.