Pour one packet of cake mix into a microwave safe mug (9 – 12 oz. mugs work best). Add 3 Tbsp. water or milk and stir until batter is smooth. Microwave on high for 1 minute and 10 seconds.
To make the streusel, combine oats, cinnamon, brown sugar, flour and salt in a medium-sized bowl. Add melted butter and use a fork our your hands to combine. It should be somewhat clumpy. Sprinkle over the cooked cake.
In a small microwave-safe bowl, microwave cream cheese for about 15 seconds. Add powdered sugar and vanilla extract. Stir until smooth, then drizzle over the streusel. Enjoy!