Step 1

Stir together the flour, sugar, baking powder, and pinch of salt in a 14 or 16-ounce mug.

Step 2

Add the milk, lemon juice, fat of choice (either melted butter or oil), and vanilla extract.  Stir the mixture together until well blended and free from lumps, making sure to scrape the bottom of the mug to make sure that all the flour is mixed in.

Step 3

Zest a fresh lemon with a microplane grater, then gently stir the blueberries and lemon zest into the batter.

Step 4

Microwave the mug cake on high for 2 minutes, or until a toothpick inserted into the center of the cake comes out clean.  Realize that the wattage of the microwave and the shape and thickness of the mug will affect how the cake cooks, so you may need to decrease or increase the time.

Step 5

Let the cake cool for 10 minutes before eating, as it is very hot.  Enjoy while warm, or cover and refrigerate for later.