In a small bowl, mix together the almond flour, coconut flour, baking soda and erythritol.
Whisk in the egg, melted butter and vanilla extract. Whisk until the batter is smooth.
Pour the batter into the prepared mug.
Microwave the cake on high power for 80 seconds. The center of the cake should be set and spring back to the touch. If the center is still soft, microwave the cake for another 10-15 seconds.
Let the cake cool for about 2 minutes then enjoy with some fresh keto whipped cream.