Add the Oreo cookie chunks to a large microwave-safe mug.
Pour the milk over the cookies, making sure they're evenly moistened. Let sit for 1 minute to soften.
Mash the Oreos with a fork until they form a thick batter. Small cookie pieces are perfectly fine.
Stir in the baking powder and optional pinch of salt until evenly combined.
If desired, place one whole Oreo on top of the batter and gently press it halfway into the center.
Microwave for 90 seconds for a fully set cake or 60 seconds for a softer, gooier center.
Top with vanilla ice cream and crushed Oreo cookies. Serve immediately.
