Choose the right mug: Use a microwave-safe mug that holds at least 10–12 ounces to prevent overflow.
Mix dry ingredients: Add almond flour, cocoa powder, sweetener, baking powder, and a small pinch of salt to the mug. Stir with a fork to break up any clumps.
Add wet ingredients: Crack in the egg, then add melted butter, almond milk, and vanilla.
Stir until the batter is smooth and no streaks remain. Scrape the bottom and sides of the mug.
Customize: Gently fold in chocolate chips, nuts, or a small dollop of nut butter if using.
Microwave: Cook on high for 45–60 seconds. Check doneness.
If the center looks too wet, microwave in 10–15 second bursts until set on top but slightly fudgy inside. Total time is usually 60–90 seconds depending on your microwave.
Rest: Let the brownie sit for 30–60 seconds. This helps it finish setting and keeps it tender.
Serve: Enjoy warm as is, or top with a spoonful of whipped cream, sugar-free ice cream, or a few fresh berries.