Ingredients 2 1/4 cups (285 g) all-purpose flour 2 1/4 cups (450 g) superfine sugar 1 cup (120 g) dark unsweetened cocoa powder (see Notes) 1 tablespoon baking soda 1 1/2 teaspoons baking powder 1 teaspoon salt 1 1/2 cups (360 ml) buttermilk, room temperature 1 cup (240 ml) hot coffee (if you can't use coffee, substitute with very hot water) 3/4 cup (180 ml) vegetable oil 2 teaspoons pure vanilla extract 3 eggs, room temperature For the frosting: 1 cup (227 g) unsalted butter, softened 1/2 cup vegetable shortening (see Notes) Pinch of salt 5 cups (620 g) confectioners' sugar 2/3 cup (160 ml) heavy cream 1 tablespoon warm water 2 teaspoons vanilla bean paste (see Notes) Bright pink food colouring (see Notes) Edible gold leaf, optional (see Notes) Pink sprinkles, optional Instructions For the cake layers: Preheat oven to 350° F (180°C). Spray the bottom of three 8-inch round cake pans with cooking spray and line bottoms with parchment rounds. Set aside.In a large mixing bowl, sift flour, sugar, cocoa powder, baking soda, baking powder, and salt.In a medium bowl or measuring cup, combine the buttermilk, coffee, oil, and vanilla, and then mix in eggs.Add liquid mixture to dry ingredients and whisk until smooth, about 1 minute. Divide batter equally among the three cake pans or weigh for accuracy--each pan should weigh about 580 grams.Bake the first two layers until a wooden pick inserted into the centre of the cake comes out with a only a few crumbs, about 23 minutes. Try not to over-bake. Let cakes cool in pans on a wire rack for 10 minutes, and then turn onto rack to cool completely. Repeat with the final cake layer. When ready to assemble the cake, wrap the layers in plastic wrap and chill for about 30 minutes. For the frosting: In the bowl of an electric mixer fitted with the paddle attachment, beat the butter, shortening, and salt on medium speed until very pale and creamy, 8 minutes.Reduce the speed to the lowest setting and gradually add the confectioners' sugar, heavy cream, vanilla bean paste and water. 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. Add a small amount of water, one tablespoon at a time, if the frosting needs more "glide." Assembly of the Watercolor Buttercream Party Cake: Transfer about 3 cups of the frosting to another bowl and tint medium pink--this will be for your cake filling and the pink on the outside of the cake. Put a smudge of frosting in the center of an 8-inch round cake board (or cake plate) or 10-inch scalloped cake board.Put your first cake layer top-up on the cake board or plate, and spread about 1 1/4 cups of the pink frosting evenly across layer. Put the second cake layer on top and repeat with another layer of frosting. Put the final cake layer top-down. Make sure that your layers are straight and lined up with the ones below.Using a turntable, if possible, frost entire outside of cake with a thin layer of the untinted frosting to seal in the crumbs. Chill until the frosting begins to firm-up, about 30 minutes. Meanwhile, tint the remaining pink frosting (there should be about 1/2 cup) a very bright pink.Remove the cake from the refrigerator and return it to the turntable. Using a medium straight palette knife (metal spatula), spread small patches of the bright pink frosting around the cake (a few inches here and there). Cover the rest of the cake with the untinted frosting as usual. Using your palette knife again, smooth the frosting around the cake, which will blend the bright pink just enough to create a "watercolor" effect. To reveal more pink, gently go over those areas again until you achieve the desired look.Fill a pastry bag fitted with a large plain round tip with the remaining frosting and pipe some buttercream "poofs" on the top. Sprinkle the top of the cake with a handful of pink and gold sprinkles. If adding gold leaf onto the cake, return the cake to the fridge to set, about 30 minutes before gently pressing random patches of the gold onto the cake. Cake will keep covered at room temperature for up to 1 day, or chilled for up to 3 days. Serve at room temperature.