Red Velvet Brownies  1 cup  unsalted butter, at room temperature  1 1/2 cups granulated sugar  1 1/2 cups brown sugar  4 large eggs, at room temperature  2 oz red food coloring (optional)  4 tsp pure vanilla extract  2 1/2 cups all-purpose flour  6 tbsp unsweetened cocoa powder  1/2 tsp salt      Preheat the oven to 350 degrees F.  Butter and flour