Stir together the dry ingredients in a 6-8 oz. microwave-safe mug.
Add vanilla, milk, vegetable oil to the mug and stir to make a batter.
In a separate bowl, use a fork or small whisk to thoroughly beat one egg. Measure out 1 tablespoon of the beaten egg and add it into the mug.
Mix in ½ of the chocolate chunks and sprinkle the other half over the top.
Microwave 1 minute - 1 minute 15 seconds. The mug cake is done when it no longer appears wet across the top of the mug cake.
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Notes
Cover tightly with plastic wrap and store on the countertop for up to 3 days.Brownie in a mug does not rise much - just a little bit. I recommend a 6-8oz. mug. Depending on your mug’s shape/size, the brownie may not come up to the top edge, but will still taste delicious.If you want more brownies, make a separate mug, do not increase the amounts in this recipe. Kitchen testing showed that increasing the amounts in the recipe do not cook without significant adjustment to the recipe and cooking time.The batter ingredients can be made right in the mug itself. I like to stir the dry ingredients together first to make sure they’re well combined. Then mix in the wet ingredients.You can use any time of chocolate chip (or chip alternate - peanut butter chips, white chocolate chips, nuts, etc.) that you’d like instead of the chocolate chunks.The egg in the recipe adds some leavening and makes a chewier brownie. This version was by far our favorite. You wouldn’t think a tablespoon of egg could make a difference in your mug cake, but it DOES! Of course, you can skip this ingredient, and you will still have a nice brownie in a mug, but if you can, add in the egg! You can make about 3 recipes from one egg.