Prep the oven and ramekins. Preheat the oven to 400°F (200°C).
Generously butter two 6-ounce ramekins, then dust the insides with cocoa powder. Tap out any excess.
Melt the chocolate and butter. Place the butter and chopped dark chocolate in a heatproof bowl. Melt gently over a double boiler, or microwave in 20–30 second bursts, stirring between each, until smooth and glossy.
Whisk in sweetener and flavorings. While the chocolate is warm (not hot), whisk in the powdered sweetener, vanilla, salt, and espresso powder if using.
Stir until dissolved and even.
Add the eggs. Whisk in the eggs one at a time until the mixture looks thick and cohesive. You want it well blended but not airy.
Fold in dry ingredients. Sift in the almond flour and cocoa powder. Fold gently with a spatula just until combined.
The batter will be silky and slightly thick.
Fill the ramekins. Divide the batter evenly between the prepared ramekins. Smooth the tops with the back of a spoon.
Bake. Place ramekins on a baking sheet and bake for 8–10 minutes. The edges should look set with a slight crust, while the centers still jiggle.
Rest and unmold. Let the cakes rest for 1–2 minutes.
Run a thin knife around the edge, then invert onto plates. If you prefer to eat them straight from the ramekins, go for it.
Serve immediately. Top with a small dollop of sugar-free whipped cream or a few berries. Enjoy while the center is molten.