In a spacious mug, whisk together the flour, sugar, cocoa powder, baking powder, and salt until smooth. Sifting the cocoa powder first can prevent clumps. This step ensures your cake has a fine, even texture and no chocolate “sand.”
Pour in milk, vegetable oil, and vanilla extract, stirring gently until everything is combined. The batter should be thick and glossy, similar to chocolate pancake batter. Fold in the chocolate chips last so they sink and form a molten center.
Cook in the microwave on high for 1 minute, checking that the edges are set while the center stays slightly jiggly. Add 10–15 second bursts if needed—overcooking will ruin the lava effect. Let it rest for a minute before diving in for that gooey, irresistible first bite.