Select a 10–12 oz microwave-safe mug. Add the wet batter ingredients: crack in the egg, add the avocado oil, and vanilla extract. Whisk thoroughly with a fork directly in the mug until uniform.
Add all the Spiced Cake Batter dry ingredients (almond flour, sweetener, flaxseed, cinnamon, cardamom, and baking powder) to the mug. Stir vigorously until the batter is smooth and no dry pockets remain. Scrape down the sides.
Microwave on high for 70–90 seconds. The cake should rise and be firm to the touch (a toothpick inserted should come out clean). While the cake cools slightly, combine the very soft cream cheese, softened butter, and powdered sweetener in a small ramekin. Whisk until smooth, then spread or drizzle over the warm mug cake.