Stir together the dry ingredients in a 6-8 oz. microwave-safe mug.
Add milk and melted butter to the mug and stir to make a batter.
In a separate bowl, use your fingers or a fork to mix together the streusel ingredients to form a crumble. Sprinkle the crumble over the top of the batter in the mug. Use a knife or fork to gently swirl the streusel just into the top of the batter.
Microwave 1 minute - 1 minute 15 seconds.
Whisk together glaze ingredients in a small bowl and drizzle over the mug cake.
Notes
Storage: Cover tightly with plastic wrap and store on the countertop for up to 3 days.Notes: The coffee cake batter will double, possibly even triple in the microwave. I would recommend using a 6-8 oz. mug so that the batter doesn’t spill over the sides of the mug. Do not double the ingredients in one mug. If you’d like a larger mug cake, make 2 or more in separate mugs. 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 substitute vegetable oil for the butter in the mug cake batter, but the mug cake will have more flavor using butter.