Choose the right mug. Use a sturdy, microwave-safe mug that holds at least 10 ounces. Anything smaller can overflow once the batter rises.

Mix the dry ingredients. Add flour, sugar, cocoa powder, baking powder, and salt to the mug. Stir well, making sure there are no cocoa clumps left at the bottom.

Stir in the wet ingredients. Add milk, oil, vanilla, and the hot water or coffee (if using). Mix until the batter is smooth and glossy. Scrape the edges so everything is incorporated.

Add the chocolate. Fold in chocolate chips if you want extra richness. Save a few to sprinkle on top for a melty finish.

Swirl the peanut butter. Drop the peanut butter in small dollops over the batter. Use the tip of a spoon or a butter knife to gently swirl it through—two or three swirls is enough. Don’t fully mix it in.

Microwave carefully. Cook on high for 60–90 seconds, depending on your microwave’s power. Start at 70 seconds, then check. The top should look set around the edges but slightly glossy and soft in the center.

Let it rest. Allow the mug cake to sit for 1 minute. It finishes cooking with residual heat and becomes extra tender.

Top and serve. Enjoy as is, or finish with a pinch of flaky salt, a drizzle of extra peanut butter, a spoon of whipped cream, or a small scoop of ice cream.