Preheat your oven to 350°F (175°C).
Grease an 8x8 inch baking pan and line it with parchment paper for easy removal.
Whisk the flour, baking powder, baking soda, and salt together in a bowl.
Beat the softened butter, oil, and sugar together until light and fluffy (about 2–3 minutes). Add the eggs one at a time, followed by the vanilla extract.
Alternate adding the dry flour mixture and the buttermilk into the wet ingredients, mixing gently just until combined.
Pour the batter into your prepared pan and bake for 22–25 minutes until a toothpick inserted in the center comes out clean. Let it cool completely.
In a medium bowl, beat the softened butter and powdered sugar until smooth.
Add the cold cream cheese and vanilla extract, whipping until completely fluffy, smooth, and spreadable.
Take your serving ramekins (ideally 4–6 oz sizes), press the open rim straight down into your cooled sheet cake, and twist. Lift up to get perfect mini cake rounds.
Drop a cake round into the bottom of each ramekin. (Optional: slice it horizontally to create two layers with frosting in between!).
Flatten the top: Spread a thick layer of frosting over the top cake layer. Use an offset spatula or butter knife to scrape across the top so the frosting is perfectly flat and flush with the rim of the ramekin.
Pour your rainbow nonpareils into a shallow bowl. Flip the ramekin completely upside down and press the flat frosting directly into the sprinkles until every single spot is covered.
Pop them in the fridge for 15–20 minutes to set the frosting. Grab a spoon, crack through the sprinkle dome, and dig in!