Aromatic Butter Base : In your mug, microwave 1 tablespoon of butter for 20-30 seconds, or until fully melted. Swirl the butter to coat the bottom and sides of the mug. Add in the lemon zest and let it sit for one minute to infuse the warm butter with a bright, citrus aroma.
Create the Batter : Into the infused butter, crack one egg and add 1 tablespoon of lemon juice and ⅓ teaspoon of vanilla bean paste. Whisk vigorously with a fork for about 30 seconds until the mixture is pale and slightly frothy.
Whisk in Dry Components : Add 1 tablespoon of almond flour, 2 ½ teaspoons of coconut flour, 3 tablespoons of the sweetener blend, ¼ teaspoon of baking powder, a pinch of sea salt, and the optional cardamom directly into the mug. Stir gently until all ingredients are just moistened into a thick batter. Do not overmix.
Fold in Berries : Gently fold in a small handful of the blueberries (about 2 ½ tablespoons) into the batter, distributing them evenly.
Microwave Transformation : Microwave the mug cake on high for 70-90 seconds. The cake is done when the top is set and springs back lightly to the touch, and a toothpick inserted into the center comes out mostly clean.
Instant Jam Topping : Carefully remove the hot mug. Immediately top the cake with the remaining blueberries (about 1 tablespoon), the remaining 1 teaspoon of sweetener, and a tiny splash of the remaining lemon juice. Use your fork to gently press and mash the berries into the hot surface of the cake, creating a fresh, warm berry compote right in the mug. Let stand for one minute before enjoying.
