• Preheat the oven to 350 degrees. Line cupcake pan(s) with cupcake liners.
• Put the first 5 ingredients into the bowl of your mixer. This is the cake flour, sugar, baking powder, baking soda, and salt. Mix on low speed for at least 30 seconds to combine.
• Add the softened butter (cut into pieces) mix on low speed just until it looks like coarse sand. The dry ingredients will be moistened/coated by the butter and will stick to together if you press some between two fingers. Don't over-mix or it will turn into a large dough-like ball.
• Add the eggs 1 at a time, mixing until blended. 
• Add the vanilla to the 1 ¼ cup buttermilk. With the mixer on lowest speed gradually pour into the mixer. Increase the speed to #2 or # 3 (on a KitchenAid mixer). Mix for about two minutes, batter will be smooth. If you are using a hand mixer rather than a stand mixer, mix a bit longer.

Making Chocolate Batter and Marbling

• In a small bowl, mix the ⅓ cup + 1 Tablespoon cocoa powder with ⅓ cup + 1 Tablespoon very hot water. Mix with a spoon until completely smooth. Allow to sit for 1-2 minutes so that cocoa flavor can develop.
• Next, put 3 cups vanilla cake batter into a medium sized bowl and gently stir in the chocolate/cocoa mixture to create chocolate batter. Set aside. Now you have approximately the same amount of chocolate batter as you do vanilla.
• Now it is time to fill the cupcake liners. Each cupcake will have about ¼ cup of batter total. Alternate between the vanilla and chocolate batter by the spoonful. Every so often, check the amount of chocolate vs. vanilla batter remaining so that you can adjust how much of each flavor you are using per cupcake if needed.
• As an optional step, once filled, you can give each cupcake a quick swirl with a toothpick or knife before baking.
• Bake at 350 degrees for about 15-17 minutes- the tops should spring back when lightly touched and an inserted toothpick should come out clean or with a few crumbs attached. Adjust time as needed.
• Move the cupcakes from the pan very soon after removing from the over. Cool them on a cooling rack.

For the Chocolate Buttercream

• Add butter and mix on low to medium speed until smooth. Add vanilla and blend into the butter. Add powdered sugar, salt, and cocoa powder.
• Add most of the milk. Begin mixing at low speed so you won't have a cloud of powdered sugar in your kitchen.
• Continue to mix adding remaining milk as needed. It may take 5 to 6 minutes to reach the smoothness and consistency you like. It will take longer if using a hand mixer. As you mix the frosting will become very smooth. You can add additional milk (1 Tablespoon at a time) if the frosting is too thick or additional powdered sugar if too soft.

Decorating the Cupcakes

• Decorate however you like! This frosting is very easy to work with. Once the cupcakes had completely cooled, we piped swirls of frosting using a disposable piping bag fitted with a large 2D star piping tip. We then added sprinkles to the top.