Heat the oven to 425 F.
Grease a 13×17” (or similar) baking sheet with 2 tablespoons 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 for 12 to 14 minutes, until the middle is set.
Brush with 2 tablespoons of melted butter and broil until golden brown. Be sure to watch carefully so it doesn’t burn.
Cut into squares, serve and enjoy!