Place 1 tablespoon of room temperature butter in mug. If cold, place in microwave for about 10 seconds. Do not melt.
Add sugar and mix well.
Add yogurt and dry ingredients, stirring until just combined.
Mixture should be thick. Smooth across the bottom of the mug.
In small bowl, combine streusel topping ingredients using a fork until well mixed. Sprinkle on top of coffee cake in mug.
Cook at full power in microwave for 1 minute. Top should be shiny, look slightly underdone and firm to touch. If additional cooking time is needed, do only 10 seconds at a time to avoid overcooking.