Preheat the oven to 350° F. Line a 12 cup muffin pan with cupcake liners.
In a large bowl combine flour, cocoa powder, baking powder, baking soda, and salt.
In a medium bowl whisk using the hand kitchen mixer eggs, granulated sugar, brown sugar, oil, and vanilla extract.
Pour half of the wet ingredients and buttermilk into the dry ingredients. Whisk between each addition using a kitchen hand mixer. Add the rest of the wet ingredients.
Spoon cupcake batter with a small (1.58 inch) ice cream scoop onto the liners.
Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean. Cool completely on a cooling rack.
Rainbow buttercream frosting
While cupcakes are cooling make buttercream frosting. Add the butter to the bowl of a stand kitchen mixer and beat for 5 minutes until doubled in volume.
Gradually add the sugar 1/2 cup at a time and beating between each addition.
Gradually add vanilla and milk. Beat on high 3 minutes, until combined.
Divide buttercream evenly into 3 bowls. Add different food coloring (pink, blue and yellow) to each mixing bowl. With a large serving spoon, add buttercream into a large piping bag. First spoon pink, blue, and yellow, continue pattern till the piping bag is full.
Frost cupcakes with unicorn rainbow buttercream icing using a jumbo size closed star piping tip. Top with unicorn sprinkles, ears, and horn from Wilton unicorn cupcake decorating kit.
This recipe makes 12 full-size cupcakes and 36 mini cupcakes. Bake mini cupcakes for 10-12 minutes.