Beat cake mix, egg, 5 Tbsp. of the butter, and 2 teaspoons of the vanilla in bowl of a heavy-duty stand mixer fitted with paddle attachment on low speed 1 minute. Increase speed to medium, and beat 2 minutes. Divide dough in half; shape each half into a 1-inch-thick disk. Wrap each disk in plastic wrap, and chill at least 2 hours or up to 24 hours.

Preheat oven to 350°F. Working with 1 disk (leave the other disk in the refrigerator), roll dough out on a heavily floured work surface. Using a  2 3/4-inch heart-shaped cookie cutter, cut out 12 cookies, and place about 1 inch apart on  a parchment paper-lined baking sheet. Repeat with remaining disk. (Reroll scraps if needed to get 24 cookies.)

Bake cookies in preheated oven just until lightly browned around edges, 6 to 7 minutes. Remove from oven, and let cool on baking sheets 5 minutes. Remove cookies to wire racks, and let cool completely, about 20 minutes.

Meanwhile, beat salt and remaining 1/2 cup butter and 1 1/2 teaspoons vanilla in bowl of heavy-duty stand mixer fitted with paddle attachment on medium speed until creamy. Gradually add powdered sugar alternately with 4 tablespoons of the milk, 1 tablespoon at a time, beating on low speed until blended and smooth after each addition. Beat in remaining 1 tablespoon milk, 1 teaspoon at a time, if needed, for desired consistency.

Frost cookies, leaving about 1/4-inch border around edges, and sprinkle with desired sanding sugars, sprinkles, and jimmies.