An adaptation from the Chocolate Root Beer Float Cake in The Sweetapolita Bakebook, this 4-layer, 6-inch round cake is a showstopper for any occasion. Four layers of super-moist, buttery chocolate root beer-infused cake filled with creamy vanilla "ice cream" frosting, topped with fluffy, satiny root-beery chocolate frosting and topped with shiny chocolate root beer glaze, scoops of vanilla ice cream frosting and sprinkles (of course).
Preheat the oven to 350℉ (180℃ ). Grease the bottoms of two 6 x 2-inch round cake pans and line with parchment, or use a baking spray alone.
In a large bowl, whisk together the flour, cocoa powder, baking powder, and salt. In a large measuring glass with a spout, combine the buttermilk and root beer. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter, shortening and brown sugar on medium speed until very light and fluffy, about 8 minutes. Add the eggs one at a time, mixing well and scraping the sides of the bowl with a rubber spatula after each addition. Beat in the root beer flavour oil or extract.
Remove the bowl from the mixer, and using a rubber spatula, fold in one-third of the flour mixture until just combined. Add half of the buttermilk mixture and fold until just combined. Repeat with the remaining flour and buttermilk mixtures. Fold in the mayonnaise.
In a small bowl, whisk together the baking soda and vinegar, and quickly fold into the batter. Don't over-mix. Divide the cake batter among the prepared pans. Bake two pans in the centre of the oven until a wooden pick inserted in the centre comes out with a few crumbs, 20 to 25 minutes (this can vary). Repeat with the final two layers. Let the cakes cool in the pans on wire racks for 10 minutes. Run a knife around the edges to loosen, and carefully turn the layers out onto wire racks. Peel off the paper, and let cool completely.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and salt on medium speed until very pale and creamy, 8 minutes. Gradually add the confectioners' sugar, heavy cream, vanilla, and melted vanilla ice cream.
Reduce the speed to the lowest setting and beat for 1 minute. Increase the speed to medium-high and beat until very light and fluffy, about 6 minutes.
The frosting will keep in an airtight container in the refrigerator for up to 3 days. Bring chilled frosting to room temperature and beat on low speed to soften. If necessary, you can warm the frosting in a heatproof container in the microwave in 10-second intervals, stirring after each one, until smooth and spreadable.
Put the butter, sugar, cream, root beer flavour oil, and salt into a food processor and pulse until combined, about 1 minute.
Melt the chocolate in the microwave. Add the melted and slightly cooled chocolate to the food processor and blend until smooth, about 1 minute. If the frosting is too soft, simply refrigerate until it firms up slight, about 10 minutes.
For ideal spreading consistency, it is best used right away. The frosting will keep in an airtight container in the refrigerator for up to 3 days. When ready to use, bring it to room temperature and beat on low speed to bring it back to spreadable consistency.
Place one of the cake layers, top up, on a cake board or plate (reserve the "sharpest" edged layer for the top). Using a small offset palette knife, spread 1 cup of the ice cream frosting on top, reaching about 1/2 inch from the edge. Repeat these steps until you come to the final cake layer, which you will place bottom up. Reserve the remaining ice cream frosting and cover in plastic wrap and chill. Wrap the cake tightly in plastic wrap. Chill for 15 minutes.
Remove the cake from the refrigerator and unwrap. Place the cake on a turntable, and frost the entire cake with a thick coat of the chocolate cloud frosting and smooth. Refrigerate for 10 minutes, and then
STOVETOP METHOD: In a medium heatproof bowl set over barely simmering water, melt the chocolate, butter, corn syrup, root beer flavour oil, and salt until smooth, stirring occasionally. Remove the bowl from the heat and let cool slightly. MICROWAVE METHOD: Add all of the ingredients to a heatproof bowl or measuring cup with spout. Microwave at high heat for 1 minute, then stir. Continue to heat in 30 second intervals until the mixture is completely melted. The glaze will keep in an airtight container in the refrigerator for up to 1 week. When ready to use, warm the glaze over a pan of simmering water or in the microwave.
Make the chocolate glaze and let it cool just enough so it won't melt the frosting but will pour freely. Pour a thin, steady stream around the top perimeter of the cake, until you have gone all the way around. Pour just enough to cover the top of the cake, pushing it to meet the edges, with your small offset palette knife. Refrigerate for about 5 minutes. Sprinkle the top of the cake with a small amount of rainbow sprinkles.
Remove the remaining Vanilla Ice Cream Bakery Frosting from the refrigerator, and use an ice cream scoop or large cookie scoop to scoop the remaining ice cream frosting onto the top of the cake. Drizzle with the remaining chocolate glaze, sprinkle with more rainbow sprinkles, and top with a cherry.
The cake will keep at cool room temperature for up to 3 days.