Preheat your oven to 350F and grease 6 cups of a standard muffin tin well with butter.
In a bowl, whisk together the almond flour, cocoa powder, granulated erythritol, baking powder, and salt. Add the egg, melted butter, and vanilla extract. Stir until a thick, fudgy cookie dough forms.
Divide the dough into 6 equal portions. Press each portion into the bottom and slightly up the sides of the greased muffin cups, creating a thick cup shape with a well in the center.
Place one square of sugar-free dark chocolate into the center of each cookie cup, pressing it gently into the dough so it sits level.
Bake for 10 to 12 minutes until the cookie edges are set but the center still looks slightly underdone. The chocolate inside will be completely melted.
Let the cookie cups cool in the tin for 5 minutes, then carefully run a thin spatula around the edges and gently lift them out. Dust with powdered erythritol and a pinch of flaky sea salt. Serve warm while the chocolate center is still molten.
