Cupcakes: Heat oven to 350. Line a 12 cup muffin tin with paper liners. Whisk flour, sugar, cocoa powder, baking soda & salt in a bowl. In large bowl, add buttermilk, oil, egg, egg yolk, food coloring, vinegar & vanilla & beat till well blended, about 2 minutes. With mixer on low, slowly beat in flour mixture till blended. Stir in 1/2 cup chocolate chips. Fill muffin cups 2/3 full & sprinkle remaining 1/4 cup chocolate chips on tops. Bake 20 minutes or till toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes, remove from pan & cool completely on rack. Frosting: Beat softened butter and cream cheese until smooth. Add confectioners' sugar and bourbon. Beat until creamy. If more Bourbon taste is preferred, add small amounts till desired taste (up to 5 Tbsp.) and if necessary, add small amounts of confectioners sugar till desired consistency is reached. The bourbon taste may be a little strong at first, but it quickly mellows out.
This frosting is one of those 'individual taste' things. It's also delicious on carrot cake, banana cake, butter pecan cake, hummingbird cake, etc.!