In a medium saucepan set over medium heat, combine the milk, 1 cup of the cream, cocoa powder, bittersweet chocolate, sugar, salt and espresso (if using). Heat until steaming, stirring frequently until smooth.

Meanwhile, whisk the egg yolks together in a large bowl. Very slowly pour the hot chocolate mixture into the egg yolks while whisking constantly until it is all combined. Pour the mixture back into the saucepan and return to medium-high heat. Cook until mixture coats the back of a spoon, while stirring constantly, about 5 minutes.

Remove from heat and stir in vanilla and the remaining 1 cup of heavy cream. If desired, pour through a mesh strainer to remove any lumps. Chill completely in the fridge, for at least 2 hours.

Freeze in an ice cream machine according to manufacturer's instructions. During the last 5 minutes of freezing, add the mini marshmallows and chopped hazelnuts. Transfer ice cream to a large bowl or plastic container. Cover and place in the freezer for at least 2 hours before serving.