Preheat oven to 350°F (175°C). Grease a 9x13-inch pan. If desired, line the pan with aluminum foil or parchment paper leaving an overhang to lift the brownies out after baking.

In a heavy-bottomed saucepan over low heat, melt 1 cup butter. Add 2 1/4 cups granulated sugar and stir continuously for about 15 seconds, until the sugar is incorporated and the mixture is smooth. Remove the pan from the heat and let the butter/sugar mixture cool briefly until it is warm but not hot (about 1–2 minutes).

In a large bowl, whisk the 4 large eggs until blended. Add 1 1/4 cups unsweetened cocoa powder, 1/2 teaspoon salt, 1 teaspoon baking powder, and 2 teaspoons vanilla extract. Whisk until the mixture is smooth and uniform.

Gradually add the warm butter/sugar mixture to the egg/cocoa mixture: pour slowly in a thin stream while whisking constantly to avoid cooking the eggs. Continue until fully combined.

Add 1 1/2 cups all-purpose flour to the batter and stir gently just until the flour is incorporated. Fold in 1 cup semi-sweet chocolate chips, mixing only until distributed.

Pour about half of the batter into the prepared 9x13 pan and spread it into an even layer.

Open the package of double-stuffed Oreos and arrange the cookies in a single layer on top of the first layer of batter. Place them close enough to cover the surface but not overlapping excessively.

Pour the remaining batter over the Oreos and spread it gently with a spatula to cover the cookies as evenly as possible.

Bake in the preheated oven for 35–40 minutes, or until the center is set and a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).

Remove the pan from the oven and let the brownies cool in the pan for at least 20 minutes. If you used foil or parchment, lift the brownies out by the overhang and place on a cooling rack to finish cooling.

Drizzle the white frosting from the tube over the cooled brownies as desired. Cut into squares and serve.