Lightly mist the inside of a microwave-safe mug or small bowl with baking spray.
Add the butter to the mug. Heat for 10-20 seconds or until melted.
Use a fork to mix in the flour, 1 tablespoon of sugar, baking powder, milk (start with 2 tablespoons and only increase if it seems super dry), and vanilla extract until the batter forms. It will look like there isn’t enough, but I promise it will fluff as it cooks.
Add the blueberries and stir just until combined.
Add the mug to the microwave and cook 1 minute to 1+1/2 minutes. Cooking time will vary depending on the wattage of your microwave so be sure to watch the muffin closely to avoid burning.