Pick the right mug: Use a microwave-safe mug that holds at least 12 ounces.
A wider mug cooks more evenly than a tall, narrow one.
Combine dry ingredients: Add 4 tablespoons flour, 2 tablespoons sugar, 2 tablespoons cocoa powder, 1/4 teaspoon baking powder, and a small pinch of salt to the mug. Stir with a fork until the color is even and no cocoa clumps remain.
Add wet ingredients: Pour in 3 tablespoons milk, 2 tablespoons oil, and 1/4 teaspoon vanilla. If you like, add a small pinch (about 1/8 teaspoon) of espresso powder.
Mix until smooth, scraping the bottom and sides of the mug. The batter should be thick but pourable.
Optional extras: Fold in 1 tablespoon chocolate chips. For a molten center, add 1 teaspoon peanut butter or Nutella on top and gently press it down into the batter.
Microwave carefully: Microwave on high for 50–70 seconds, depending on your microwave’s power.
Start with 60 seconds. The cake should rise, look set on top, and feel soft when gently pressed. If it’s wet in the center, heat in 5–10 second bursts until just done.
Let it rest: Allow the cake to sit for 1–2 minutes.
It will finish cooking with the residual heat. This short rest also improves texture.
Top and serve: Dust with powdered sugar, add a scoop of ice cream, or drizzle with chocolate sauce. Enjoy straight from the mug while warm.