Whisk together the yolk, cream and vanilla in a small bowl.
Place the flour, sugar, ground walnuts and salt in a food processor and process briefly to combine. Add the cold butter pieces and pulse to cut the butter in until the mixture resembles coarse meal with pea-sized lumps, about 8 brief pulses.
With the food processor still running, add the liquid egg mixture and process just until the dough comes together around the blade. Dough should be soft and tender.
Turn the dough onto a sheet of plastic wrap and press into a 6-inch disk. Wrap the dough in the plastic and refrigerate at least 2 hours.
Remove the dough from the fridge. Cut into six equal pieces and roll each into a ball. Wrap in plastic wrap and refrigerate all but one ball.
Working one at a time on a lightly floured surface, roll or press each ball flat, then press into the tartlet tins. Dough will be fairly thick. Roll the rolling pin over the top of the tart pan to remove the excess dough and use the scraps to patch any tears.
Continue with the other mini-tarts, then lightly pierce the bottoms of each tart with the tines of a fork. Freeze all of them in their pans for 30 minutes.
When ready to blind bake the crusts, preheat oven to 375°F. Set the tarts on a baking sheet and line each shell with foil or parchment squares. Fill with pie weights (rice, beans, ceramic beads, whatever you use) and bake for 15 to 18 minutes, or until bottom crust doesn’t stick to foil or parchment.
Remove from the oven and carefully remove the parchment paper and weights. Beat the egg white then brush the tart crusts with it - this will help keep them sealed against leaks. Continue to bake the tart crusts until a golden brown, another 10 to 15 minutes. Let cool.