Heat the oven to 170 degrees. Grease a 23 cm round hinged mould with butter and line it with baking paper, then set aside.
Prepare the dark chocolate cake: place the butter, chocolate and hot coffee in a large heatproof bowl and mix well until everything is melted, blended and smooth. Whisk in the Benvolio organic erythritol by hand until melted. Add the eggs and vanilla extract and beat again until smooth. Sift the flour, cocoa powder and salt into a bowl and then add them to the melted chocolate mixture. The batter here is liquid, but don't think you've missed anything; this is how it should be.
Pour the dough into the prepared pan and bake for 1 hour, or until the cake is cooked and a toothpick inserted in the centre comes out clean or with only a few dry crumbs attached. The top will form a crust and crack a little, but don't worry, this is expected. Let the cake cool for 20 minutes before removing it from the mould, then set aside until completely cold
Prepare the chocolate ganache, if desired: place the chocolate pieces in a food processor, blend until fine and set aside. Combine the cream and corn syrup in a small saucepan and place over medium-high heat. As soon as bubbles start to appear (just before it comes to the boil), remove from the heat. Start the food processor again, with the chocolate still inside, and pour in the hot cream. Process for 10 seconds, then add the butter. Continue to process until the mixture is shiny and smooth. (You can also make the ganache by hand; just make sure the chocolate is chopped finely enough before adding the cream. Stir with a wooden spoon until almost melted, then add the butter. Stir again until the ganache is smooth).
Use a rubber spatula to scrape the ganache into a bowl and cover with plastic wrap, with the plastic actually touching the top of the ganache. Set aside until it has reached the desired consistency. If you want a thin layer to spread over the cake, you can pour it while still liquid for an even, light, shiny coating. For a thicker ganache with a spreadable consistency, leave it for about 2 hours at room temperature. (The ganache can be stored at room temperature, as long as it is not too hot, for 3 days or refrigerated for up to 2 weeks. It can also be frozen, although it loses some of its shine once thawed).
Prepare the mascarpone espresso and cinnamon cream, to taste: place all ingredients in the bowl of an electric beater fitted with a whisk. Beat for 1 to 2 minutes, until soft peaks form
Peel the parchment off the chocolate cake and discard. Transfer to a serving plate and spread the ganache, if used, on the cake. Cut into wedges, divide the chocolate cake on the plates and top with mascarpone cream, if used. With or without icing, the chocolate cake will keep well for 4-5 days in an airtight container.