First Step: Prepare your baking pan and oven. Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan or a 10×15 inch jelly roll pan if you prefer thinner brownies. For easy removal, line the pan with foil and lightly spray the foil with non-stick cooking spray.

Second Step: Mix wet ingredients. In a large bowl, whisk together 1 cup melted unsalted butter and 2 tablespoons vegetable oil until well combined. Stir in 1/2 cup unsweetened cocoa powder until the mixture is smooth and uniform in color.

Third Step: Combine dry ingredients. In a separate bowl, mix 2 cups all-purpose flour with 1/2 teaspoon salt, 1 cup granulated sugar, and 1 cup packed dark brown sugar. Gradually stir the dry ingredients into the cocoa mixture until just combined.

Fourth Step: Incorporate eggs and vanilla. Whisk 4 large eggs, one at a time, into the batter, mixing well after each addition. Then add 1 tablespoon vanilla extract and stir gently to combine. Avoid overmixing to maintain a thick, fudgy batter.

Fifth Step: Bake the brownies. Pour the batter evenly into the prepared pan and spread with a spatula. Bake in the preheated oven for 25-30 minutes for the 9×13 pan or 15-20 minutes for the jelly roll pan. The brownies are done when the surface looks matte and bubbly, and a gentle touch in the center feels slightly firm but springy. A fork inserted should come out mostly clean with some moist crumbs.

Sixth Step: Cool and frost. Allow brownies to cool about 15 minutes in the pan. Meanwhile, prepare the frosting by beating 1/4 cup softened butter, a pinch of salt, and 1/4 cup unsweetened cocoa powder until smooth. Gradually add 2 1/2 to 3 cups powdered sugar, alternating with 1/4 cup milk and 1 teaspoon vanilla extract, adjusting to desired consistency. Spread the frosting evenly over the warm brownies for a soft, silky finish. If desired, add rainbow sprinkles on top to brighten presentation. Let the brownies cool completely before slicing.