1. Bake the cake. Prepare the cake mix according to the package directions, adding an extra 1/4 cup of vegetable oil if using a boxed mix for extra moisture. Bake in a 9x13-inch pan until done, then let the cake cool in the pan for about 15 minutes. Turn it out into a large mixing bowl and cool until just slightly warm.
2. Crumble the cake. Once the cake is cool enough to handle, use clean hands to break it into fine crumbs. Rub larger pieces between your fingers until the texture resembles soft, damp sand with no big chunks.
3. Make the frosting. In a separate bowl, beat the softened butter, icing sugar, vanilla, and milk together for about 3 to 4 minutes, until light, fluffy, and smooth. The frosting should be spreadable but not runny.
4. Combine cake and frosting. Add about 3/4 of the frosting to the cake crumbs. Mix by hand or on low speed until the mixture holds together like soft cookie dough when pressed. If it feels dry and crumbly, add a spoonful or two more frosting; if it feels wet or greasy, add a few extra crumbs.
5. Shape and chill the cake balls. Scoop tablespoon-sized portions of the mixture and roll into smooth balls. Arrange on a parchment-lined tray. Chill in the freezer for 10 minutes, then re-roll each ball to smooth any cracks. Chill again for 30 minutes in the freezer or 1 to 1 1/2 hours in the refrigerator until very firm.
6. Melt the coating. Place about half of the chocolate or candy melts in a microwave-safe mug or deep bowl. Microwave in 20-second bursts, stirring between each, until completely melted and smooth. If using regular chocolate, stir in a small splash of oil to thin to dipping consistency if needed.
7. Attach sticks and dip. Dip the tip of a cake pop stick about 1/2 inch into the melted chocolate, then insert it halfway into a chilled cake ball. Let the chocolate seal for a minute. Holding the stick, dip the cake pop into the melted chocolate, turning to coat. Gently tap off excess coating and immediately decorate with sprinkles or toppings. Place upright in a styrofoam block, box with holes, or rice-filled glass to set.
8. Let set and serve. Repeat with the remaining cake balls, reheating or adding more melted chocolate as needed. Allow all cake pops to set completely at cool room temperature or in the refrigerator until the coating is firm. Serve chilled or at room temperature. Store leftovers covered in the fridge for up to 5 days or freeze for longer storage.
