• Preheat the oven to 350 degrees F and grease a 9x13 baking pan well with butter or nonstick spray.
• Combine the milk and vinegar in a measuring cup or bowl and allow it to sit for 5 minutes until slightly curdled.
  1 1/2 cups whole milk, 1 tablespoon vinegar
• In a large bowl, whisk together the flour, sugar, baking powder, and salt until fully combined.
  3 cups all-purpose flour, 1 3/4 cups granulated sugar, 1 tablespoon baking powder, 1/2 teaspoon salt
• Add the oil, vanilla extract, and egg into the milk mixture and whisk together until smooth.
  1/2 cup neutral oil, 1 tablespoon vanilla extract, 1 large egg
• Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.
• Fold the rainbow sprinkles gently into the batter. Avoid using nonpareils in the batter because the colors bleed heavily.
  1/2 cup rainbow sprinkles
• Pour the batter into the prepared 9x13 pan and smooth the top evenly.
• Bake for 35-40 minutes or until a toothpick inserted into the center comes out with minimal crumbs.
• Remove the cake from the oven and allow it to cool completely. I wait about an hour.
• Use a biscuit cutter or round cookie cutter to cut circles from the cake. You can also form the scraps of the cake to make layers as well.

Buttercream

• To make the buttercream, beat the softened butter until fluffy using a hand mixer or stand mixer.
  1 1/2 cups butter softened
• Gradually add the powdered sugar, vanilla extract, and milk. Continue beating until smooth and creamy. Add more milk if needed for consistency.
  5-5 1/2 cups powdered sugar, 4-5 tablespoons milk, 2 teaspoons vanilla extract
• Place one cake round into a dessert cup and spread frosting on top.
• Add another cake round and top with additional frosting.
• Pour rainbow nonpareils into a bowl and gently dip the frosted tops upside down into the sprinkles until fully coated.
  1 cup rainbow nonpareils
• Serve immediately or chill until ready to serve.