Prepare the mug: Lightly grease a 12-ounce microwave-safe mug (or small ramekin) with a little butter or coconut oil to prevent sticking.
Mix dry ingredients: In the mug (or in a small bowl if you prefer), whisk together almond flour, cocoa powder (or chopped baking chocolate), sweetener, baking powder, and a pinch of sea salt until evenly combined and no lumps remain.
Add wet ingredients: Stir in the egg, melted butter, vanilla extract and heavy cream (or milk). Mix until you have a smooth, glossy batter — thick but pourable. If using chopped chocolate for a lava center, gently press it into the center of the batter so it sinks slightly.
Microwave bake: Microwave on HIGH for 90 to 120 seconds. The top should look set but still slightly glossy; avoid overcooking — you want the center molten. If your microwave is powerful, start checking at 75 seconds.
Let it rest briefly: Let the mug cake sit 30–45 seconds after cooking — this helps the edges set while the center remains gooey.
Serve immediately: Top with sugar-free whipped cream or coconut cream, a few sugar-free chocolate chips or a sprinkle of nuts, and fresh berries if desired. Dig in while warm and gooey.