Combine the cocoa powder, flour, sugar, and salt in a 10- to 12-ounce microwave-safe coffee mug. Using a fork, stir until well combined. Add the milk, oil, and vanilla. Stir just until the dry ingredients are almost combined and the mixture thickens slightly, about 15 seconds. Add the chocolate chips, and stir until well combined. Sprinkle a few more chocolate chips on top, if desired.

Place the mug in the microwave and microwave on high power for about 1 minute. (This time can vary based on how powerful your microwave is. Adjust for more or less time as needed once you make one.) The brownie should puff slightly and still appear shiny and moist in spots on the top. Continue cooking in 10-second intervals if the batter still looks raw or oily. Let the brownie cool for a few minutes before serving with ice cream.