Preheat oven to 375ºF and spray a 9x13 sheet pan with nonstick cooking spray.
Add melted butter to a mixing bowl and slowly whisk in buttermilk. Mixture will become slightly thick. Whisk in beaten egg, sugar, and vanilla extract until well combined.
On top of wet ingredients, add flour, baking powder, baking soda, and salt. Toss together gently before stirring all ingredients together until just combined.
Gently fold in 2 tablespoons of sprinkles. Don’t overmix or they may bleed their color.
Spread into prepared sheet pan in an even layer. Sprinkle with remaining 1 tablespoon sprinkles.
Bake for 12 minutes or until set in the middle. Cut into squares and serve with desired toppings.