In a 12-ounce or larger microwave-safe mug, add 4 Oreo cookies and milk and mash until well-combined, about 1 minute. Stir in sugar, flour, cocoa powder, and baking powder.
Microwave 1 minute. Serve with ice cream or whipped cream and garnish with an Oreo cookie (whole or broken into pieces).
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Notes
Unsweetened cocoa powder: Natural or Dutch-processed. Dutch-processed cocoa offers a darker color and a bit richer flavor, but either tastes incredible in this cake. Note that this is different than hot cocoa powder.
Yield: This recipe makes 1 (4-ounce) mug cake plus ice cream or whipped cream.
Storage: This cake is best enjoyed the day it is made.