In a microwave-safe mug, add 3 tablespoons granulated sugar, 3 tablespoons all-purpose flour, 1 ½ tablespoons unsweetened cocoa powder, and a pinch of salt. Stir with a small whisk or fork until well combined and free of lumps.
Stir in 2 tablespoons of chocolate chips.
Pour in 1 ½ tablespoons vegetable oil (or melted coconut oil or melted butter), 3 tablespoons milk, and ¼ teaspoon vanilla extract. Stir with a spoon or small rubber spatula until just combined. Do not overmix.
Sprinkle a few extra chocolate chips on top of the batter.
Microwave the mug on high for 60 seconds. Check to see if the brownie is done (the top should look set, and it’s okay if the chocolate chips are still melty). If needed, continue microwaving in 10-second increments until there is no raw batter.
Let the brownie cool for 1–2 minutes before eating.
Enjoy straight from the mug, or top with ice cream, whipped cream, powdered sugar, or berries as desired.