Choose the right mug: Use a microwave-safe mug that holds at least 12 ounces to prevent overflow.
Mix dry ingredients: Add oat flour, cocoa powder, baking powder, and salt to the mug. Stir well to break up any cocoa clumps.
Add wet ingredients: Stir in maple syrup, milk, Greek yogurt (or applesauce), oil, and vanilla until smooth.
Scrape the edges and bottom so no dry spots remain.
Fold in chocolate: Mix in chocolate chips. Save a few to sprinkle on top for a melty finish.
Microwave carefully: Cook on high for 45–60 seconds. Check the center.
If it’s still very wet, continue in 10-second bursts until just set. The top should look slightly glossy but springy.
Rest briefly: Let it stand for 1 minute to finish setting and cool to a comfortable temperature.
Serve: Enjoy as is, or top with a dollop of yogurt, a few berries, or a drizzle of nut butter.