Place the tablespoon of sugar-free caramel sauce in a small silicone mold or on a piece of parchment paper and freeze for at least 20 minutes until it firms up into a solid disc or nugget.
In a microwave-safe mug (at least 12 oz), whisk together the almond flour, coconut flour, brown sugar substitute, baking powder, and salt until evenly combined.
Add the egg, melted butter, heavy cream, and vanilla extract. Stir well until the batter is smooth and uniform with no dry spots.
Spoon half the batter into the mug, place the frozen caramel piece right in the center, then cover completely with the remaining batter so the caramel is hidden inside.
Microwave on high for 70 to 80 seconds, until the cake looks set around the edges. The center should still feel slightly soft, which means the caramel pocket inside is perfectly gooey.
Let the mug cake rest for 30 seconds. Top with a drizzle of sugar-free caramel sauce, a dollop of whipped cream, and a generous pinch of flaky sea salt. Dig in immediately so you hit that warm, oozy caramel center.
