Preheat oven to 350°F (175°C). Grease and line one 6-inch round pan.
Mix cake batter per box directions. Pour into pan and bake 22–28 minutes until a toothpick comes out clean. Cool 20 minutes, then remove from pan.
Beat butter until smooth. Add powdered sugar, vanilla, and cream. Beat 3–4 minutes until fluffy. Tint if you like.
Trim cake top flat. Crumb coat with a thin layer of buttercream. Chill 10 minutes.
Use an open star tip to pipe ruffles in rows around the cake, starting at bottom and working up. Chill 10 minutes before serving.