• Preheat oven to 350°F and line a 12-cup muffin/cupcake pan with paper liners.
• In a small bowl lightly combine the egg yolks, half the milk, and orange zest.
• Combine the flour, sugar, and baking powder and mix on low speed for 30 seconds. Add the butter and remaining milk. Mix on low speed until the dry ingredients are moistened. Increase the mixer speed to medium and beat for about 2 minutes, until light and fluffy. Scrape down the sides of the bowl. Gradually add the egg mixture, in 3 additions, beating about 30 seconds after each addition to incorporate the egg.
• Divide batter into cupcake pan, filling about halfway. Bake 15-18 minutes, rotating in the oven halfway through, or until a toothpick inserted into the center of the cake comes out clean. Let cool completely before frosting.

For the frosting:

• In a microwave safe bowl, melt butter. Using a hand mixer, mix in cocoa powder until completely mixed, breaking up any clumps.
• Alternate adding powdered sugar and milk in three additions, mixing completely and scraping down the sides after each addition. Add more powdered sugar or milk to reach desired consistency. Mix in vanilla extract.
• Spread onto cooled cupcakes or fill a piping bag and pipe frosting in desired pattern.