1 box white cake mix (plus ingredients needed to make the cake according to the box instructions)
2 sticks (8 ounces) unsalted butter, at room temperature
4 cups confectioners' sugar (from a 1-pound box)
Pinch fine salt
2 teaspoons vanilla extract
2 tablespoons milk
1 ounce (about 1/4 cup) semisweet chocolate chips
12 regular-sized marshmallows
12 chocolate-covered peppermint candies, such as York Peppermint Patties