'The Bear' Chocolate Cake

Divide batter evenly among prepared baking pans (about 3 cups per pan); tap baking pans a few times on work surface to evenly spread batter and eliminate any air bubbles. Bake in preheated oven until a wooden pick inserted in center of cakes comes out clean, 25 to 30 minutes. Let cake layers cool in baking pans on wire racks 10 minutes; remove cakes from baking pans and let cool completely, parchment side down, on racks, 1 hour and 30 minutes to 2 hours.

Make the icing

Sift together powdered sugar and cocoa in a medium bowl. Set aside. Beat butter with a stand mixer fitted with the paddle attachment on medium speed until creamy, about 2 minutes. Gradually add powdered sugar mixture, beating on low speed just until combined, about 2 minutes,

 scraping down sides of bowl as needed. Increase mixer speed to medium and beat until fluffy, 2 to 3 minutes. Gradually add cream and vanilla, beating on medium-low speed until combined and stopping to scrape down sides of bowl as needed. Spoon 1 cup icing into a piping bag or ziplock plastic bag; cut a 1/2- to 3/4-inch hole in corner. Leave remaining icing in bowl.

Assemble the cake

Trim cakes flat using a cake leveler or serrated knife, if necessary; discard parchment. Place one cake layer on a cardboard cake round or plate. Pipe a 1/2-inch-tall border of icing around edge of first cake layer. Spoon half of chocolate mousse (about 2 cups) onto center of cake, and spread evenly within border using a small offset spatula. Top with second cake layer, cut side down, and repeat process with remaining chocolate mousse. Top with third cake layer, cut side down. Spread remaining icing over top and sides of cake. Chill until set, about 15 minutes.