Take a large microwavable mug. Through a sieve, sift the dry ingredients like all-purpose flour, powdered sugar, cocoa powder and baking powder.
Now add milk and vegetable oil to the cup. Use a fork or a spoon to give a good mix. Now dunk the chocolate pieces into the mug and cover them using the batter.
Now microwave the mug for about 3 minutes. Check by inserting a toothpick. If it comes out clean, your brownie is done. If not, then microwave for 1-2 minute more, as per the need.