Preheat the oven to 325°F. Grease an 8-inch springform pan and line the bottom with parchment paper.
Melt the chocolate and butter together in the microwave in 30-second bursts, stirring between each, until smooth. Let it cool for 5 minutes.
Blend the cottage cheese until silky smooth. Add the eggs, sweetener, cocoa powder, espresso powder, vanilla and salt and blend until combined.
With the blender on low, pour in the melted chocolate mixture and blend just until the batter is glossy and uniform.
Pour into the prepared pan and smooth the top. Bake for 30 to 35 minutes, until the edges are set and the center has a slight wobble.
Cool completely in the pan, then refrigerate for at least 2 hours to firm up. Serve chilled or at room temperature, dusted with cocoa powder or topped with whipped cream.