How to Make It
Prepare cake according to package instructions for a 9x13-inch cake. Cool cake 20 minutes.
Using a wooden dowel or the round handle of a wooden spoon, poke holes over the entire cake surface; be careful to not poke all the way to the bottom of the cake pan.
Place the chopped white chocolate in a medium glass bowl. Heat the heavy cream in a small saucepan over medium-high heat until simmering (do not bring to a boil). Immediately pour the hot cream over the chopped chocolate; whisk until chocolate is melted and mixture is smooth. Whisk in the peppermint extract. Pour the warm chocolate mixture over prepared cake, being sure to fill the holes with the melted mixture, and smoothing any excess chocolate mixture evenly over the surface of the cake. Let cake stand 30 minutes.
Beat butter and salt at medium speed with an electric mixer for 1 to 2 minutes or until creamy. Gradually add powdered sugar alternately with 6 tablespoons milk, beating at low speed until blended and smooth after each addition. Stir in vanilla. If desired, beat in remaining 1 tablespoon milk, 1 teaspoon at a time, until frosting reaches desired consistency. Spread the mixture evenly over the cake; sprinkle evenly with chopped buttermint candies.