Prepare vanilla cake according to box directions (or use your favorite homemade recipe), folding in 1/3 cup rainbow sprinkles into the batter before baking. Bake in a 9x13-inch pan and cool completely to room temperature, at least 1 hour.

Crumble the cooled cake into fine, uniform crumbs in a large mixing bowl using your hands, breaking up any large pieces until the texture resembles coarse sand.

Add frosting 1 tablespoon at a time, mixing thoroughly with your hands after each addition, until the mixture holds together when squeezed but isn't wet or greasy (you'll likely need 6-8 tablespoons total).

Roll the mixture into 24 evenly-sized balls, about 1.5 tablespoons each, rolling firmly between your palms to create smooth spheres. Place on a parchment-lined baking sheet.

Refrigerate the cake balls for at least 2 hours or freeze for 30 minutes until very firm to the touch.

Melt candy melts according to package directions in a narrow, deep bowl or measuring cup, stirring in coconut oil if the coating seems too thick for smooth dipping.

Remove 6-8 cake balls from the refrigerator at a time to keep the rest cold. Dip each lollipop stick about 1/2 inch into melted coating, then insert it halfway into a cake ball. Return to the baking sheet and repeat with remaining balls, then chill for 10 minutes to set the sticks.

Holding a cake pop by the stick, dip it into the melted coating at a slight angle, submerging completely. Gently tap the stick against the bowl's edge to remove excess coating, rotating slowly to let drips fall back into the bowl.

While the coating is still wet, sprinkle with additional rainbow sprinkles or decorations if desired. Immediately stand the pop upright in a styrofoam block or prepared holder.

Let the coating set completely at room temperature for 15-20 minutes, or refrigerate for 10 minutes to speed the process. Store in an airtight container in the refrigerator until ready to serve.