Preheat oven to 180°C (350°F). Line an 8x8 inch baking pan with parchment paper.
Twist apart the Oreo biscuits and remove the cream filling.
Make the batter
Blitz the Oreo biscuits in a blender until a fine powder forms with no visible chunks.
Transfer to a bowl, add baking powder, and whisk to combine.
Pour in milk gradually, stirring to form a thick, smooth batter. Add extra milk one tablespoon at a time if the batter looks too dry.
Fold in chocolate chips or walnuts if using.
Bake
Pour batter into the prepared pan and smooth the top.
Bake for 18-20 minutes until a toothpick comes out with moist crumbs (not wet batter).
Cool in the pan for 15 minutes before slicing into 9 squares.
Notes
Grind the biscuits to a very fine powder for the smoothest texture. Coarse crumbs make crumbly brownies.Do not overbake — these look slightly underdone at 18 minutes but firm up as they cool. Pull them out when the center still has a little jiggle.Cool completely before cutting for clean slices. Rushing this step results in crumbling.Use parchment paper to prevent sticking since there is no butter in the batter.Store in an airtight container at room temperature for up to 2 days, or in the fridge for up to 4 days. They get fudgier on day two.