Prep your mug or ramekin. Use a microwave-safe mug (10–12 oz) or a 6–8 oz ramekin. Lightly grease the inside with butter or a neutral oil so the cake slides out easily.

Crush the Oreos. Add the Oreos to a zip-top bag and crush with a rolling pin until they’re fine crumbs with no big chunks. Or place them in the mug and crush with the back of a spoon. You want a sandy texture.

Make the batter. In the mug, stir in milk until the crumbs turn into a thick, pourable batter. Add melted butter, cocoa powder, sugar, baking powder, salt, and vanilla. Mix until smooth. The batter should be slightly thicker than pancake batter. If it’s too thick, add a splash more milk.

Create the lava center. Press a well into the middle of the batter with a spoon. Add the chocolate squares or chips to the well, then spoon a little batter over the top to cover it completely. This is what gives you that molten center.

Microwave in short bursts. Microwave on high for 45–60 seconds to start. The cake should rise and set at the edges but still look slightly glossy on top. If needed, add 10–15 second bursts until the edges are set but the center is soft. Most microwaves land between 60–90 seconds in total.

Rest before serving. Let it sit for 30–45 seconds. The carryover heat finishes the structure without overcooking the lava center.

Top and enjoy. Add ice cream, whipped cream, or a dusting of powdered sugar. Eat warm with a spoon. For a plated look, carefully loosen the edges with a knife and invert onto a small plate.