Preheat the oven to 350°F (175°C). Line a 9x9-inch pan with parchment paper, letting a bit hang over two sides for easy removal later. Lightly grease the parchment.

In a small saucepan or microwave-safe bowl, melt the butter until warm and fully liquid. Let it cool slightly. Stir in the sugar until combined.

Whisk in the eggs, one at a time, then add vanilla. Whisk until smooth and glossy.

Add the cocoa powder, buttermilk (or milk + vinegar), baking powder, salt, and red food coloring. Stir until combined.

Gently fold in the flour until just combined.

Fold in most of the chopped sandwich cookies and the white chocolate chips, leaving a handful to sprinkle on top.

Pour the batter into the prepared pan and smooth the top. Sprinkle the remaining cookie pieces on top.

Bake for 22–28 minutes or until a toothpick inserted near the center comes out with a few moist crumbs.

Let the brownies cool completely in the pan on a wire rack. Use the parchment to lift out and slice into squares.