Preheat the oven to 425 degrees F.
Grease a 17x13” (or similar) baking sheet with 2 tablespoon butter.
In a large bowl, whisk together the flour, cornstarch, powdered sugar, baking powder, and salt.
In a separate bowl, whisk together the buttermilk, eggs, melted butter, and vanilla extract.
Add the wet ingredients to the dry ingredients and mix with a spatula until just combined. Don’t overmix!
Pour into the prepared baking sheet.
Bake in the preheated oven for 12-14 minutes, until the middle is set.
Brush with 2 tablespoon of melted butter and broil until golden brown. Be sure to watch carefully so it doesn’t burn!
Cut into squares, serve, and enjoy!