In a microwave-safe mug, whisk together almond flour, cocoa powder, erythritol, baking powder, and a pinch of salt until well combined.

Add melted butter, heavy cream, vanilla extract, and the egg to the dry ingredients. Stir thoroughly until the mixture forms a smooth batter.

Ensure the batter is evenly distributed in the mug to allow for even cooking.

Microwave the mug on high for 60-75 seconds, depending on the wattage of your microwave. Start with 60 seconds and check; the brownie should be firm but slightly moist in the center.

Let the mug cake cool for 1-2 minutes before serving. Optionally, top with a dollop of whipped cream or a sprinkle of sugar-free chocolate chips for added indulgence.