I've adapted this recipe for a cookie in a mug by making it into a banana bread and making it gluten-free. However, if you're not restricted to eating gluten-free, I have a feeling it would be just as delicious replacing the gluten-free flour in the recipe with normal flour. It makes for a decadent, cakey treat that's sweet, gooey, and comforting.
1 tablespoon melted butter
1 egg yolk
1 tablespoon sugar
Scant 1/4 cup gluten-free flour*
1/2 very ripe banana, mashed
Dash of salt
Few drops of vanilla extract
Mix all ingredients with a fork in the bottom of a microwave-safe mug. Microwave for 45 to 60 seconds (mine is perfect at 55 seconds, but all microwaves are different) until the banana bread is cooked through. Eat it while it's hot and enjoy every bite.