Chocolate Ganache Cake
- With a serrated knife, trim tops of cake layers to level. Place bottom layer on a wire rack set over a parchment-lined rimmed baking sheet. Spread 1/4 cup frosting on top of layer. Place second layer, cut side down, on top. Spread a thin layer of ganache frosting over cake to form a crumb coat; refrigerate until firm, about 30 minutes.
- Slowly pour ganache glaze over top and down sides of cake (use an offset spatula to spread evenly, if needed). Refrigerate until set, about 15 minutes. Scrape excess glaze into a bowl, passing it through a fine sieve to remove crumbs. Pour a second coat of glaze over cake; refrigerate until set, about 15 minutes.
- Divide buttercream into 4 portions; tint to desired shades with gel-paste food coloring (we used the colors above to mix 2 shades each of yellow and peach). Fit 4 pastry bags with couplers and fill each with a different tinted buttercream.
- Mark a line across the center of the cake's surface with a toothpick to serve as a guide. Attach an open-star tip, such as Ateco #864 or Wilton #4B, to a pastry bag and fill with buttercream. Hold the bag at a 90-degree angle, gently squeeze, then release halfway into each peak, drawing the tip up until the icing forms a point. Pipe a line of stars across the center, following the line, then add more rows, working from the center out and changing colors every few rows to create an ombre effect.