Place 1 TBSP of butter in your coffee mug (average 6 oz. mug works fine!). Microwave for about 10-15 seconds until softened (NOT melted).
Mix in 2 TBSP of sugar and 1/2 beaten egg.
Stir in 1/4 cup flour, vanilla yogurt, and 1/8 TSP baking powder. Voila! You already have an amazing cake in a mug ready to go. But now...the best part.
In a small bowl, mix together 2 TBSP flour, 1 TBSP brown sugar and 1 TSP cinnamon until well combined. Add 1 TBSP butter and mix well until you get coarse crumbles (yup, I use my hands!).
Pour crumbles on top of your cake.
Microwave for about 1 minute then 10 second increments until cake is fully cooked.