Make the streusel first. In a small bowl, combine the almond flour, brown sugar substitute, and cinnamon. Add the cold butter pieces and pinch everything together with your fingers until you get coarse, crumbly pieces. Mix in the chopped pecans and set aside.
In a microwave-safe mug (at least 12 oz), whisk together the almond flour, coconut flour, granulated erythritol, baking powder, cinnamon, and salt.
Add the egg, melted butter, sour cream, and vanilla extract. The sour cream is the secret here as it makes the cake incredibly moist and tender. Stir until the batter is smooth and creamy.
Scatter the streusel topping evenly over the batter. Do not press it in or stir it. You want it sitting right on top so it stays crumbly.
Microwave on high for 70 to 80 seconds, until the cake beneath the streusel is set and springs back when lightly touched. The streusel on top should look golden and slightly toasted.
Let the mug cake rest for 1 minute before eating. The streusel will crisp up slightly as it cools, giving you that perfect buttery crunch on top of the soft, moist cake.
