Preheat the oven to 350 degrees Fahrenheit. Line two cookies sheets with parchment paper and set aside. Add cake mix, eggs and oil to a medium sized mixing bowl and mix until well combined.
Add the powdered sugar, vanilla, salt and 1 T of milk to the bowl. Beat until well combined. Add more milk, 1/2 Tablespoon at a time, until the frosting reaches a consistency where it can be easily spread, but still retains it's shape.
Cut the large marshmallows in half diagonally with a sharp kitchen shears or chef's knife. Place the pink sanding sugar into a bowl and dip the freshly cut side of the marshmallow into the pink sugar to create the bunny ears.
Immediately, while frosting is still wet, place a heart shaped sprinkle in the center of the bottom 1/2 of the cookie. This will create the nose on the cookie.
Add two of the mini marshmallow halves directly below the heart to create the bunny muzzle. Then place two of the large marshmallow halves at the top of each cookie to create the ears. Then place two black pearl sprinkles between the ears and nose to create the bunny eyes.