Place dry ingredients (flour, sugar, cocoa powder, salt, and baking powder) in a large microwave-safe mug. Whisk to combine.
Add milk, oil, and vanilla extract; stir to combine well.
Fold in the chocolate chips and walnuts.
Place mug in the microwave and cook for 60 seconds on high power.
Check the mug brownie—if batter is still wet, cook for another 30 seconds. Usually, they remain just a bit raw in the centre after the first 60 seconds, and that’s a clue to stop; if not, your brownie could end up overcooked and dry.
Notes
You can make a dairy-free version of this mug cake by swapping the milk for almond or soy milk.