What is it
A mug brownie, or brownie-in-a-cup, is a single serving of brownies made in a mug and cooked in the microwave. They take just a few minutes to make, are vegan, and very tasty.
Mix equal parts flour, sugar, cocoa powder, oil, and water in a mug. Microwave for a minute. Done.
i.e. in a big mug
1/4 cup flour
+ 1/4 cup sugar
+ 1/4 cup water
+ 1/4 cup cocoa (this is unsweetened baking cocoa, not hot chocolate mix)
+ 1/4 cup oil
50 ml flour
+ 50 ml sugar
+ 50 ml water
+ 50 ml cocoa
+ 50 ml oil
You can easily halve the recipe for smaller servings
• a pinch of salt brings out the chocolate flavor
• add a dash of vanilla extract and/or almond extract and/or peppermint extract add/or a few shakes of cinnamon
• add 1/4 tsp (1 or 2 ml) of baking powder to make it a bit fluffier
• replace some (or all) of the flour with the same amount of cocoa powder for an extra chocolate-y brownie
Who is it for
Mug brownies are for anyone with a sweet tooth, but the activity of making them is for anyone who can manage a measuring spoon. 3+ is probably right.
What Kids Like
Kids are motivated by the speed of the process. From saying, “Let’s make brownies!” to actually having them in your mouth can take as little as three minutes. Cooking is a great activity to do with kids, but they often get bored or frustrated having to wait.
What Parents Like
Cooking is a great way to teach some basic math (how many teaspoons in a tablespoon?  how many milliliters in a quarter cup? [~60]) as well as basic cooking concepts (mix the dry ingredients before adding the wet, leveling the measuring spoons before dumping it) and this recipe lends itself to some scientific inquiry. For example, what happens if we add half the sugar, or half the oil, or use brown sugar?
Now, teaching kids to wait, and be bored for a bit is actually an essential concept these days, with instant access to almost anything, and these brownies are the food equivalent of on-demand streaming media, but they are fun and the benefits outweigh the drawbacks.