Combine all ingredients except streusel ingredients into oversized mug. Stir with a small whisk or fork until batter is smooth. Make sure your mug is very big or else the streusel will slide right out during cooking. You can divide batter into two mugs, but then I recommend doubling the streusel recipe.
Make streusel in small bowl with fork until butter is mixed with the flour and sugar and resembles a coarse topping.
