In a microwave-safe mug, stir together the granulated sugar, all-purpose flour, and unsweetened cocoa powder until the mixture is lump-free.
Add the chocolate chips, vegetable oil, milk, and vanilla extract. Stir gently until the batter is just combined; avoid overmixing for a fudgy texture.
Sprinkle a few more chocolate chips on top, then microwave on high for 60 seconds. Check the center and, if needed, microwave in 10-second bursts until set but still moist.