Making the Layer Cake

• You must use a round cake pan to make a yin and yang birthday cake. Bake two cakes of equal size, one vanilla flavored and the other chocolate flavored. Because the cake can take some time to assemble and it often needs to be made in advance, keep your cakes fresh by brushing the tops of each with cooled simple syrup. Make the syrup by dissolving 1 cup sugar in 1 cup water and simmering for 3 minutes. Once the cakes have cooled, slice each cake in half, creating two half-moon shapes each of vanilla and chocolate cake.

Making the Icing

• Make a stiffer buttercream icing — use more sugar than corn syrup — to ensure that the icing design on the cake does not run. However, use a softer icing for sandwiching the layers of the cake, to keep a light texture inside. For a yin and yang cake, you need both black and white icing. To make black icing, start with white buttercream icing, and add cocoa powder in a 4-1 ratio of icing to cocoa powder. If necessary, thin out the icing and cocoa mixture with some water as the extra powder can cause the icing to stiffen. Add roughly 1 ounce of black food coloring to every 2 cups of darkened buttercream icing. Adjust the amount of cocoa and coloring to taste. Too much black coloring can create a bitter taste, so it is important to darken the icing with cocoa.

Assembling the Layers

• Spread the soft buttercream on top of 1 half-moon, before placing its matching half on top, creating a two-tiered half-moon cake in a single flavor. Repeat for the other cake, creating a second half-moon cake. Place strips of parchment paper on your cake tray and, using a large spatula, carefully transfer one half-moon cake to the tray. Ice the cut side of the cake with a little buttercream before carefully bringing up the second half-moon cake to the tray. The two half-moon shapes should sit with their cut sides facing each other. You will have a circular, two-tiered cake, one half of which is vanilla — white — and the other chocolate — black.

Icing the Outside

• Before icing your cake, draw the wavy, curving center line of the yin and yang symbol down the middle of your cake using a thin line of icing. Frost half the cake, the side that is predominately vanilla cake, with white icing, and the other side with black icing. Use a spatula to create a smooth appearance, with no streaks. Take a round plastic bottle cap and push it gently onto the wide half of the black icing, centering it. This will be the outline for the opposite-colored dot. Repeat on the other side, wiping off the lid before reusing to reduce smudging. Fill in the circle outlines on each half with icing of the opposite color, so that there is a black icing circle in the white icing half and vice versa.