Stir the flour, cocoa powder, sugar, and salt in a small bowl until combined. Add the milk, oil, and vanilla. Give it a few stirs with a fork or whisk, and then add the chocolate chip, stir or whisk until completely combined. Transfer batter to a 10-12 ounce microwave-safe mug and top with additional chocolate chips.
Microwave on high for 1 minute. The brownie will have risen but should still have some moist spots on the top. If the batter looks uncooked, microwave in 10-second intervals until done. Let it cool in the mug for a few minutes before serving with vailla ice cream or whipped cream.