In a microwave-safe mug or heatproof cup (at least 10 oz capacity), add 1/4 cup all-purpose flour, 2 tablespoons granulated sugar, 1 tablespoon unsweetened cocoa powder, 1/4 teaspoon baking powder, and a pinch of salt. Stir with a fork or small whisk to combine thoroughly and break up any lumps.

Add wet ingredients

Make a small well in the center of the dry mixture and add 3 tablespoons milk, 1 tablespoon vegetable oil, and 1/4 teaspoon vanilla extract. Stir until just combined and smooth, scraping the bottom and sides so no dry streaks remain. If using chocolate chips, fold in 1 tablespoon now.

Microwave cook

Microwave the mug on high power for 60 to 90 seconds. Start checking at 60 seconds: the cake should have risen and be set on top but still moist. If it still looks wet or jiggly in the center, continue microwaving in 10-second intervals until set. Avoid overcooking or the cake will become rubbery.

Cool and serve

Let the cup cake cool for 2 minutes (it will be very hot). Optionally dust with 1 tablespoon powdered sugar or top with whipped cream. Serve warm directly in the mug.