Mix together flour, sugar, baking powder, and salt in a microwave-safe mug.
Stir in milk, vanilla, and the melted butter.
Crush Oreo cookies with your hands and stir into mug cake batter.
Scrape down the sides of the mug, then wipe down the sides and rim before baking so that your mug cake cooks evenly.
Microwave for 90 seconds on high.
Let cool for a minute or two.
Top with whipped cream and more crushed Oreos, if desired.
Notes
A straight-sided mug makes it easier for the cake to rise.
Mix your batter until just combined, and don’t overmix.
Let the cake cool a little before eating, because it is quite hot when it comes out! This also lets the batter continue cooking.
Test out this recipe with your microwave: all microwaves are different, and you may find you need more or less time to bake your mug cake. And of course, if you like a gooey cake, you can reduce the cooking time a little.