12 Tbsp unsalted butter, melted, plus more for baking pan 2 cups flour, plus more for dusting baking pan 1 1/2 tsp baking powder 1/4 tsp kosher salt `1 1/4 cups brown sugar 1/2 cup white sugar 1 tsp vanilla extract 2 large eggs, lightly beaten 2 cups white chocolate, roughly chopped, or white chocolate chips
Heat oven to 350. Grease and flour a 9×13 baking dish
Whisk together flour, baking powder and salt. Set aside.
Whisk together the brown and white sugars, vanilla and eggs. Whisk until smooth. Add melted butter and continue stirring until smooth. Change implement from a whisk to a large spoon or baking spatula and stir in dry ingredients, about a third at a time, fully incorporating. The batter will become very dense.
Stir in white chocolate and distribute evenly throughout the batter. Pour batter into prepared pan and bake until a toothpick inserted in the center comes out clean with a few crumbs, about 20 minutes. Cool on a rack.