
Preheat the oven to 350F degrees. Line a 9x13 inch pan with parchment paper, or aluminum foil and lightly grease.
In a large bowl, whisk together the melted butter and sugar (or beat with an electric mixer.
Whisk in the eggs and vanilla extract. You don't want the butter to look like it's melting out at this point.
Stir in the flour, cornstarch and salt, ensuring that there are no flour clumps.
Fold in the chocolate chips/ mix-ins.
Spoon the batter into the prepared pan and smooth the top.
Bake in the preheated oven for 24-28 minutes, or until the top looks set and shiny and an inserted toothpick comes out clean or with a few damp crumbs.*
Cool fully before slicing.