Put 2 tablespoons butter and sugar in a small mixing bowl and stir until well combined.
Add egg, sour cream and vanilla and stir.
Mix together 1/2 cup flour flour and baking powder and add to mixing bowl. Stir again.
In a separate small bowl, mix remaining butter, 4 tablespoons flour, brown sugar and cinnamon until crumbly.
Divide half of the egg mixture between two mugs. Divide half the cinnamon mixture and sprinkle into the mugs. Top with the remaining egg mixture, and then with the remaining cinnamon mixture.
Microwave each mug (one at a time) for 1 minute and 15 seconds. The cake will puff up about an inch or so while it is cooking, so if you are using small mugs it will puff up over the top and if you are using regular sized mugs it won’t – either is fine!
Let the cakes cool for a few minutes and then dig in with a spoon!