Prepare the Chocolate Base : In your medium microwave-safe mixing bowl, melt the unsalted butter. This should take approximately 30-40 seconds on high power. Add the cocoa powder, powdered sweetener, and sea salt directly to the melted butter. Whisk vigorously until a thick, glossy paste forms.
Temper and Combine : Add the room-temperature eggs to the chocolate paste one at a time, whisking thoroughly after each addition to ensure a smooth, emulsified batter. Pour in the heavy cream and vanilla extract, and continue to whisk until the batter is uniform and slightly aerated.
Portion the Batter : Using a spatula, scrape the sides of the bowl and give the batter one final stir. Carefully divide the batter evenly among the three ramekins or mugs. Each should be about half to two-thirds full.
Create the Molten Core : Divide the chopped sugar-free dark chocolate into three portions. Gently press one portion into the center of the batter in each mug. Submerge the chocolate pieces just below the surface; they will sink further as the cake cooks.
Microwave to Perfection : Cook the cakes one at a time in the microwave. Cook on full power for 75 to 90 seconds. The ideal cake will be set and have pulled away from the sides of the mug, but the very center will still look soft and slightly wet.
Rest and Serve : Let the lava cake rest for one minute before serving. This allows the center to set slightly and the molten lava to form. Enjoy immediately, topped with keto-friendly whipped cream, fresh berries, or a dusting of powdered sweetener. For a dramatic presentation, carefully invert the cake onto a small plate.
