Stir together the dry ingredients in a 6 to 8-ounce microwave-safe mug.
Add the 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 for 1 minute to 1 minute 15 seconds.
Whisk together glaze ingredients in a small bowl and drizzle over the mug cake.
