How to Make It
In a food processor, pulse the flour, sugar and salt. Add the butter and pulse until the mixture resembles small peas. Sprinkle the egg yolk mixture on top and pulse until the pastry comes together in large clumps. Turn the pastry out onto a work surface, knead 2 or 3 times and pat into a disk. Wrap in plastic and refrigerate just until chilled. Let stand at room temperature for 10 minutes before rolling.
On a lightly floured work surface, roll out two-thirds of the pastry to an 11 1/2-inch round. Roll the pastry around the rolling pin and transfer it to a 9-inch fluted tart pan 1 inch deep with a removable bottom. Press into the pan, folding in the overhanging dough to reinforce the sides. Trim the overhang and knead the scraps into the remaining dough.
On a floured work surface, roll out the remaining pastry to a 9 1/2-inch round. Using a pastry wheel, cut the round into 3/4-inch-wide strips.
Preheat the oven to 350°. In a bowl, toss the blackberries, sugar and flour; spoon into the tart shell. Scatter the butter on top. Arrange the strips over the berries in a lattice pattern, pressing the ends onto the pastry rim. Trim any excess pastry. Brush the lattice with the egg yolk mixture; sprinkle with the turbinado sugar.
Bake the tart on a sheet of foil in the bottom third of the oven for about 1 hour, or until the pastry is golden and the juices are bubbling. Transfer to a wire rack to cool for at least 3 hours.
In a bowl, beat the cream and sugar to soft peaks. Add the crème fraîche and vanilla and beat until firm peaks form. Cut the tart into wedges and serve with the crème fraîche.
Turbinado sugar is unrefined so it is a light tan color and has a coarser texture than granulated sugar. "Sugar in the Raw" is probably the most commonly available brand.