In a microwave-safe mug, combine applesauce, nut butter, maple syrup and vanilla extract. Whisk until smooth.
Once smooth, add oat flour, almond flour, baking powder, a pinch of salt and milk. Stir until combined; then fold a tablespoon chocolate chips into the mug cake batter.
Place the mug in the microwave and cook on high for approximately 1 minute, or until the cake has risen and is set in the center. Microwaves may vary, so adjust the cooking time accordingly.
Carefully remove the mug from the microwave (it will be hot!) and allow the cake to cool for a minute or two before serving.