Make the filling: in a small pot over medium-low heat, add the butter and chopped chocolate. Once melted, sift in the cocoa and powdered sugar. Whisk to combine into a smooth ganache and pour into a bowl to allow to cool while you make the scone dough.
Make the scone dough: Whisk together the flour, baking powder, salt and sugar. Add the butter and use a pastry cutter to combine it into the flour so that the butter cubes are kidney bean-sized. You can also use a cheese grater to slice in the butter (see video for guidance).
In a separate bowl, add the heavy cream, egg and vanilla and whisk to combine. Pour this mixture into the flour & butter mix and use a spatula to stir.
Switch to your hands and squeeze and fold the dough over itself to until it mostly comes together in a ball.
Roll it out to about 20x12 inches , and spread the chocolate ganache over it, then sprinkle the hazelnuts on top. Roll the dough into a long log and place on a parchment lined cookie sheet and in the freezer.
Once mostly frozen, after about an hour or two, slice into V shaped scones (the bottom of the V should be a bit wide, as pictured). You can bake them now or store in an airtight container in the freezer for later baking.
To bake the scones: Preheat the oven to 400 F. Line a cookie sheet with parchment paper and place the scones, leaving room for them to spread. Brush heavy cream onto the tops and sprinkle granulated sugar on the top.
Bake for about 20 minutes until the tops and edges are golden brown – don’t overbake.