Preheat oven to 350°F. Grease and line one half sheet baking pan with parchment paper.
In a large bowl, at medium speed, beat together butter and sugar until light and fluffy. Add the eggs, one at a time, and the vanilla. Continue mixing until well incorporated. Scrape the bowl as needed.
In a separate bowl, whisk flour, baking powder, and salt. With the mixer at low speed, alternately add flour mixture and milk. Ending the process with flour mixture. Mix about 1 minute. Add sour cream and mix until incorporated.
Fold in rainbow sprinkles. Pour the batter evenly into the prepared baking sheet and bake for 25-30 minutes or until a toothpick inserted in the middle of the cake comes out clean. Do not turn off the oven just yet.
Remove the cake from the oven and cool in a wire rack. Crumble the cake into a large bowl.
Line one baking sheet with parchment paper and add 2 cups of cake crumbles to it. Distribute evenly and bake for 20 minutes or until golden brown. Remove from oven and cool completely. Once cooled, process in a food processor until the cake crumbles resemble sand. Pour mixture in a shallow dish and set aside.
Mix milk and vanilla and combine with the cake crumbles. Toss and mix until the texture of the mixture is compactable enough to form a ball. You may add additional milk if needed.
Mix melted chocolate with oil.
Using a small ice cream scoop, form balls. Roll each ball between the palms of your hands. Shape until smooth and round.
Using disposable gloves, put 1 tablespoon of melted white chocolate in the palm of your hand. Roll a ball of cake and drop into the shallow bowl containing the crumbles sand. Roll each ball until completely coated with the mixture. Place in mini cupcake liners and serve.