Preheat oven to 350°F (177°C). Line the bottom and sides of a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides to lift the finished brownies out (makes cutting easier!). Set aside.

Prepare brownie batter. Spread into prepared baking pan. The homemade brownie batter is very thick, so do your best to spread it to the edges. Doesn’t have to be perfectly neat. Press 24 Oreos down into the top of the brownie batter; you want 6 rows of 4 Oreos.

Bake for 30 minutes, then test the brownies with a toothpick. Insert it into the center of the pan. If it comes out with wet batter, the brownies are not done. If there are only a few moist crumbs, the brownies are done. Keep checking every 2 minutes until you have moist crumbs. My homemade brownies with Oreos on top take about 35 minutes.

Remove from the oven and place on a wire rack to cool completely in the pan. Once cool, lift the parchment paper out of the pan using the overhang on the sides. Do not cut into squares yet. (TIP: It’s helpful to place the whole slab of brownies (parchment paper still underneath) on a baking sheet because you will need to chill the layers. The baking sheet makes transporting to the refrigerator easier!)

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, milk, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 2 full minutes. With a spoon or rubber spatula, fold in the chopped Oreos. Taste. Add a pinch of salt if frosting is too sweet. I like adding 1/8 teaspoon salt. Spread buttercream on top of cooled brownies. Chill uncovered for 30 minutes and up to 1 day in the refrigerator.

Cut the butter into a few pieces and place in a small saucepan with the chocolate chips. Melt the two together on low heat, stirring constantly, until smooth and combined. You can also use the microwave—stop and stir every 20 seconds until melted and smooth.

Pour chocolate on top of the buttercream. Spread all over the top in an even layer. Allow chocolate to set at room temperature for a few hours or in the refrigerator for 1 hour before slicing and serving. See recipe notes for my tips for cutting neat brownie squares.

Cover and store leftover brownies in the refrigerator for up to 1 week.