Add flour, cocoa powder, baking powder, sugar, and salt to a microwave safe mug. Mix.
In a separate dish, combine milk and flaxseed to create a “flax egg”. Let sit for ~2 minutes.
Add flax egg, oil, and vanilla extract to the dry ingredients and stir until just combined.
Microwave on high for 60-75 seconds, or until slightly puffed up and top springs back when lightly touched.
Serve immediately with fresh berries, powdered sugar, and/or a dollop of your favorite plant-based ice cream or whipped cream.