Chocolate Unicorn Cake

Ingredients

• Chocolate Cake:
• 1 3/4 cups all-purpose flour
• 1 cup dark cocoa powder (such as Hershey's(R) Special Dark), sifted
• 2 teaspoons baking soda
• 1 teaspoon baking powder
• 3/4 teaspoon salt
• 1 cup firmly packed brown sugar
• 1/2 cup white sugar
• 1/2 cup unsalted butter, softened
• 2 large eggs, at room temperature
• 2 teaspoons vanilla extract
• 1 cup strongly brewed coffee, cooled to room temperature
• 1 cup sour cream
• Vanilla Frosting:
• 1 cup unsalted butter, softened
• 4 cups sifted powdered sugar
• 1 1/2 teaspoons vanilla extract
• 1/4 teaspoon salt
• 1 tablespoon heavy cream, or more as needed
• Decorations:
• 1 drop blue food coloring, or as needed
• 1 drop purple food coloring, or as needed
• 1 tablespoon pink sprinkles, or as needed
• 1 (.75 ounce) tube black decorating gel
• 4 marshmallows, quartered
• 1 tablespoon pink sanding sugar, or as needed (optional)
• 8 birthday candles