Mix the oat flour, cocoa powder, baking powder, and salt in a microwave-safe mug until well combined.
Add the milk, maple syrup, and vanilla extract. Stir until you get a smooth, thick but pourable batter. Scrape the bottom to ensure no dry pockets remain.
Drop the melted chocolate or chocolate chips into the center of the batter, letting it sit slightly submerged.
Microwave on high for 45–60 seconds. Start checking at 45 seconds; the top should be set but the center soft.
Let the cake rest for 1–2 minutes before eating to stabilize the texture.