Pick the right mug: Use a microwave-safe mug that holds at least 12 ounces. A wider mug helps the cake cook more evenly.

Melt the butter: Add 1 tablespoon of butter to the mug. Microwave for 15–25 seconds until melted. Let it cool for a few seconds so it doesn’t scramble the egg.

Whisk in wet ingredients: Add 1 egg and 1/2 teaspoon vanilla to the melted butter. Whisk with a fork until smooth.

Stir in the dry mix: Add 3 tablespoons almond flour, 1 1/2 tablespoons unsweetened cocoa powder, 1 1/2 to 2 tablespoons keto sweetener (adjust to taste), 1/4 teaspoon baking powder, and a small pinch of salt. Mix until no dry pockets remain. The batter should be thick but spoonable.

Make the lava center: Press one square of sugar-free chocolate into the center of the batter, pushing it down slightly and covering it with a little batter. Alternatively, add 1 tablespoon sugar-free chocolate chips to the middle.

Microwave: Cook on high for 40–60 seconds. Start checking at 40 seconds. The top should look just set but still glossy. If it’s soupy at the edges, microwave in 5–10 second bursts until done. Avoid overcooking to keep the center molten.

Rest and serve: Let the mug cake sit for 45–60 seconds to finish setting. Enjoy straight from the mug or run a butter knife around the sides and invert onto a plate.

Optional toppings: Add a dollop of unsweetened whipped cream, a sprinkle of cocoa powder, or a few berries for contrast.