Preheat oven to 275 °F and spray 9″ springform pan with oil of choice.
Combine chocolate and butter in a bowl. Melt in a microwave at 50% power, stirring occasionally, ~4 min. Let cool for 5 min.
Whisk together eggs, erythritol, water, glucomannan, vanilla, and espresso powder. Whisk in in chocolate mixture until smooth and slightly thickened. Filter through fine-mesh strainer into pan, then gently tap pan on counter to remove air bubbles.
Let cake mixture sit for 10 min. to let air bubbles rise to the top. Pop bubbles with a fork, then bake for 45-50 min., until edges are set and center jiggles slightly when shaken.
Let cool for 5 min., then run a paring knife around the sides of the pan. Let cool on a wire rack until barely warm, ~30 min., then cover, poke holes in top, and refrigerate for at least 6 h.
Remove sides of pan and use an offset spatula to separate cake from bottom tray. Let stand at room temperature for 30 min., then serve with whipped cream.