Chocolate Unicorn Cake
1 (.75 ounce) tube black decorating gel 1 cup dark cocoa powder (such as Hershey's® S… 1 cup firmly packed brown sugar 1 cup sour cream 1 cup strongly brewed coffee, cooled to room … 1 cup unsalted butter, softened 1 drop blue food coloring, or as needed 1 drop purple food coloring, or as needed 1 tablespoon heavy cream, or more as needed 1 tablespoon pink sanding sugar, or as needed 1 tablespoon pink sprinkles, or as needed 1 teaspoon baking powder 1 ½ teaspoons vanilla extract 1 ¾ cups all-purpose flour 2 large eggs, at room temperature 2 teaspoons baking soda 2 teaspoons vanilla extract 4 cups sifted powdered sugar 4 marshmallows, quartered 8 birthday candles ¼ teaspoon salt ½ cup unsalted butter, softened ½ cup white sugar ¾ teaspoon salt