In a 12-oz microwave-safe mug, melt butter in the microwave for about 30 seconds.
1 tablespoon salted butter, melted
Then, crack an egg into the mug, along with the milk and vanilla extract. Whisk all together with a fork.
1 large egg, 3 tablespoons milk, ¼ teaspoon vanilla extract
In a separate bowl, stir together all dry ingredients until well mixed.
1 tablespoon all purpose flour, 4 tablespoons granulated sugar, 2 tablespoons cocoa powder, ¼ teaspoon baking powder
Add dry ingredients to the mug and mix everything together with a fork, until all ingredients are well blended.
Microwave for 90 seconds. The cake batter will rise and expand as it cooks, then will settle down as it’s cooling. The top of the cake should be moist to touch.
Serve warm, but let cool for several minutes before enjoying.