In a large microwave-safe mug, whisk together the almond flour, coconut flour, granulated erythritol, and baking powder until no lumps remain.
Add the egg, heavy cream, melted butter, vanilla extract, and almond extract. Whisk vigorously with a fork until the batter is completely smooth.
Gently fold in the sugar-free rainbow sprinkles with a spoon, being careful not to overmix so the colors don't bleed.
Microwave on high for 70 to 80 seconds. The cake should be risen and set on top with a slightly moist center. If still wet, microwave in 10-second bursts.
Let the mug cake cool for 1 to 2 minutes. Top with sugar-free vanilla frosting and a scattering of extra sprinkles. Eat straight from the mug with a spoon.
