Melt the Chocolate: Start by melting the butter and no-sugar-added dark chocolate in a microwave or over a double boiler until smooth and shiny.
Whisk the Eggs: In a separate bowl, beat the egg, egg yolk, erythritol and vanilla extract until light and a bit frothy.
Combine Mixtures: Fold the melted chocolate mixture into the whisked eggs, stirring to combine.
Add Dry Ingredients: Mix in the almond flour, cocoa powder, and a small pinch of salt until you have a thick batter.
Prepare Ramekins: Grease 2 ramekins with butter and divide the batter between the two.
Bake: Transfer into a preheated oven at 375°F (190°C), bake for 10-12 minutes or until edges are set and centers are still slightly soft.
Rest and Serve: Allow cakes to rest for 2-3 minutes; carefully invert onto plates. Serve hot with whipped cream or powdered cocoa.