In a microwave-safe mug (ideally 12 oz or larger to prevent overflow), whisk together the flour, sugar, cocoa powder, and baking powder. This dry blend is the foundation of your cake’s texture, so whisk thoroughly to remove any lumps and ensure even mixing.
Add the milk, oil or melted butter, vanilla extract, and peppermint extract (if using). Stir gently but firmly until you achieve a smooth, lump-free batter. This step is crucial—lumpy batter equals an uneven, sad cake. Aim for a consistency that’s thick but pourable.
Place a microwave-safe plate or napkin over the top of the mug to trap steam and prevent splatter. Microwave on high for 1–1.5 minutes, depending on your microwave’s wattage. Start with 1 minute, then check for doneness. Avoid overcooking—it can turn your mug cake rubbery.
A toothpick or skewer inserted in the center should come out mostly clean. If the batter still looks wet, microwave in 15–30 second intervals until done. The cake should rise and slightly deflate—that’s normal and a good sign of a tender crumb inside.
Allow the cake to cool for 1–2 minutes. It will be piping hot and continues to cook slightly as it sits. Now comes the fun part—toppings. Try any of the following for a decadent finish:
Whipped cream
Mini chocolate chips
Caramel or fudge drizzle
Crushed peppermint or cookie crumbs
Scoop of vanilla ice cream