Preheat the oven to 325 F. Grease an 8-inch round springform pan and line the bottom with parchment paper. Place the chopped chocolate and butter in a heatproof bowl set over a pot of simmering water. Stir gently until completely melted and smooth. Remove from heat and let cool for 5 minutes.
Whisk the sugar into the chocolate mixture until dissolved. Add the eggs one at a time, whisking well after each. Stir in the vanilla extract.
Sift the cocoa powder and tapioca starch over the chocolate mixture, add the salt, and fold in gently until smooth and no dry pockets remain.
Pour the batter into the prepared pan. Bake for 25 to 30 minutes until the edges are set and the center has just a slight wobble. The torte will firm up as it cools. Let cool completely in the pan, then refrigerate for at least 2 hours.
Remove the torte from the springform pan. Whip the heavy cream to soft peaks and spread it over the top, or serve a generous dollop alongside each slice. Dust with cocoa powder just before serving.