In a microwave-safe mug (at least 12 oz), whisk together the almond flour, cocoa powder, granulated erythritol, baking powder, and salt until there are no lumps of cocoa remaining.
Add the egg yolk, melted butter, heavy cream, and vanilla extract. Stir vigorously with a fork until the batter is completely smooth and glossy. The batter should look thick and fudgy, not cakey.
Fold in the sugar-free chocolate chips, pressing a few down into the batter so they melt inside during cooking.
Microwave on high for 50 to 60 seconds. The top should look barely set while the center still appears slightly underdone and jiggly. This is exactly what you want for that fudgy, molten texture.
Let the mug cake rest for 1 full minute before eating. The residual heat will continue cooking it gently while keeping the center dense and gooey. Sprinkle with a pinch of flaky sea salt and enjoy immediately with a spoon straight from the mug.
