Mix the Dry Ingredients - In a medium bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt.
Whisk in the Wet Ingredients - Add milk, vegetable oil, egg, and vanilla. Whisk until smooth and just combined.
Pour into the Mugs - Evenly divide the batter into two 14-ounce microwave-safe mugs. Add about 1 cup per mug. Adjust so it’s no more than halfway filled.
Add Chocolate Chips - Evenly sprinkle 1 tablespoon of chocolate chips on top of the batter.
Microwave the Cakes - Cook one mug at a time on high power for 60 seconds. Check to see if the cake sets. The top will look moist but dry to the touch. It should bounce back when lightly pressed, and a toothpick inserted in the center has only a few crumbs sticking. If needed, microwave in 15-second intervals until cooked through. I usually don't cook longer than 90 seconds. The internal temperature should reach between 200 to 210ºF (93 to 99ºC). The higher the temperature, the less gooey the cake.
Serve - Rest for 5 minutes, then serve. If desired, top with extra chocolate chips and powdered sugar.
Notes
Recipe Yield: 2 cups of batter.
Smaller portions: Divide 1/2 cup-sized portions into 4 mugs.
Filling: Don’t fill the mug more than halfway with batter, or you may have a mess.
Make it Gluten-Free: Use 1:1 gluten-free baking flour like Bob's Red Mill or King Arthur.
Make it Dairy-Free: Substitute dairy milk with coconut, almond, soy, or oat milk.
Make it Vegan: Omit the egg and use plant-based milk instead of dairy.
Oil Substitution: Vegetable oil can be substituted with melted butter, melted coconut oil, avocado oil, or olive oil.
Storing: Do not store unbaked batter, as the baking powder will lose some of its leavening ability. Cool cooked cake completely, then cover with foil for up to 2 days. Reheat in the microwave on high in 15-second intervals until warm.
Microwave Watts: This recipe was developed on a 950-watt microwave. If needed, adjust the time up or down 15 seconds depending on the strength of your system.