Preheat oven to 375ºF and spray a 9 x 13 inch 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.
Spread into prepared sheet pan in an even layer.
Bake for 12 minutes or until set in the middle. Cut into squares and serve with desired toppings.
Notes
Can make with regular milk, increase baking powder to 2 teaspoons, omit baking soda.