In a food processor, combine flour, hazelnuts, sugar, and salt. Pulse to combine. Add the cubed butter and pulse until a coarse crumb-like mixture forms. Add 1 tablespoon of water and pulse until the dough starts to form a ball. If you find the dough to be too dry, you can add more water, 1 teaspoon at a time. Turn out the dough onto a work surface and form into a disc. Wrap in plastic wrap and chill in the refrigerator until firm, at least one hour.
Once dough has chilled, preheat the oven to 350˚F. Lightly butter six 4-inch individual tart tins and set aside.
On a generously floured work surface, roll the dough into a rough circle, about 1/8-inch thick. Make sure to work quickly, as the dough is easiest to work with while cold. Cut three 6-inch circles from the dough. Gather the remaining scraps into a disc, roll out again and cut three more circles from the dough. Line the tart tins with the cut circles, being careful not to stretch the dough, as it will shrink when baked. Roll a rolling pin over each tin to trim any excess dough. Place the prepared tins in the fridge to chill.
While the tart shells chill, prepare the filling. Using a stand or hand-held mixer, whisk together the eggs and sugar on medium-high speed until pale and fluffy, about 2 minutes. With a spatula, fold in the walnuts and melted butter. Remove the tart shells from the refrigerator and divide the filling between them. Place the tins on a sheet pan and bake in the oven until the filling is golden brown and the crust is cooked through, about 15 – 20 minutes. If you find the tarts are browning too fast, cover them with aluminum foil while they finish baking. Remove from the oven and transfer to a wire rack to cool slightly while you make the sauce.
To make the sauce, in a medium saucepan, melt the butter, brown sugar and cream over medium heat. Stir with a wooden spoon until smooth. Once bubbling, leave the sauce without stirring for 3 minutes. Remove from the heat and add the walnuts and stir until smooth. Allow to cool until just warm before serving.
To serve, remove the tarts from their tins and transfer to serving plates. Top each tart with a scoop of vanilla ice cream, if desired, and a drizzle of the butterscotch sauce.