Preheat oven to 400 degress F.
Prepare a 13" x 18" jelly roll pan by brushing the pan with the melted butter and lining it with parchment paper before brushing the remaining butter on top.
2 tablespoons melted butter
In a medium bowl, sift together flour, cocoa powder, and salt.
½ cup unsweetened cocoa powder, 2 tablespoons all-purpose flour, ½ teaspoon kosher salt
In a large bowl, whip together eggs and sugar until the mixture becomes lighter in color.
5 large eggs, ⅔ cup white sugar
Add vanilla extract and slowly whisk 1/3 of the dry flour & cocoa mixture.
½ teaspoon vanilla
Add another 1/3 of the mixture, whisk slowly, and add the final 1/3 until just mixed. Do not overmix.
Pour batter into prepared baking pan and spread evenly with a spatula.
Bake for 8 - 10 minutes until the top is dry and the edges begin to lift.