Measure your ingredients using a kitchen scale. It's the most accurate and will give the most consistent results.
In a large bowl, stir together the flour and salt.
Grate the frozen butter with a box grater and toss it directly onto the flour mixture, mixing it up as you go. After all the butter is grated, use a pastry blender to cut it into the flour until coarse, pea-sized crumbs appear. Set aside.
Whisk the sourdough starter and egg yolk together. Pour the liquid into the flour mixture and fold in until the dough just holds together. Squeeze a small amount of dough between your fingers and if it is very crumbly, add some ice water, 1 tablespoon at a time (2 tablespoons maximum). Try not to overmix the dough...you want to keep that butter cold and separate from the flour.
Turn dough out onto a lightly floured surface and push together into a rough ball. Knead a few times to combine, then divide into two equal balls (if making a tart, leave whole). Flatten each ball into a disc with smooth edges (no cracks), cover with plastic wrap, and refrigerate at least 1 hour or up to overnight. If you refrigerate the dough for more than 30 minutes, you may have to rest it for a few minutes at room temperature before it will be soft enough to roll out.
Lightly dust your rolling surface with flour. Use even pressure to roll the dough out front to back and on the diagonals. Turn and loosen the dough occasionally as you continue to roll the pastry out into a circle about 2 inches larger than your pan with a thickness of approximately ¼-inch (about the height of two stacked quarters), unless otherwise directed by your recipe.
Transfer the dough to the pan you're using. Ease the crust into the pan, pressing up on the sides and leaving the dough overhanging the rim.
For a tart pan: Use your rolling pin to roll over the rim to create a neat edge.
For a pie pan: Trim the overhang to 1-inch, then turn the overhang inward and crimp as desired.
Continuing: Save the dough scraps to repair the crust later if needed. Prick the bottom all over with a fork.
To blind bake the crust: Preheat the oven to 375°F. Tear off a piece of foil that fits over the entire pan with overhang (or add extra foil as needed). Spray the foil with baking spray to keep it from sticking to the dough. Press the foil into the pan and tightly up against the sides. Bring the foil up over the rim and mold it to the sides of the pan. Freeze the crust for 15 minutes. You don't need to use pie weights or dried beans using this frozen-foil method.
Bake the crust with the foil on for 20 minutes. At this point the crust is par-baked.
Carefully remove the foil from the crust. If the crust has puffed up, gently press it back down. Any tears or cracks in the crust can be repaired with dough scraps. Return the crust to the oven and bake the crust until golden brown, 5 to 10 minutes more.
If you prefer to use chilled butter instead of frozen, cut it into small cubes and toss it into the flour.You can use a standing mixer fitted with the paddle attachment or a food processor to make the dough in Step 3. Just be mindful to pulse the ingredients quickly and don’t overwork it. You want that butter to stay cold and to keep the flour from developing too much gluten so the crust will be flaky.Shortcrust pastry dough can be frozen after Step 5. Just defrost the dough overnight in the refrigerator before proceeding with Step 6.For mini tartlets, spray the cups of two mini muffin pans with baking spray. Roll out the chilled shortcrust pastry and cut rounds using a 2½-inch round cookie cutter. Using a pastry dough tart tamper, push the rounds into the muffin pan cups. Chill the dough for 15 minutes in the freezer or 30 minutes in the refrigerator to firm up (you don’t want slumped tartlet shells). Bake for 10 minutes at 400°F for par-baked shells or 20 minutes for fully baked shells. Allow to cool before proceeding with filling.The leftover egg white can be whisked with a teaspoon of water and a pinch of salt for use as an egg wash before par-baking. It won’t result in quite as golden a crust as a whole-egg wash, but it’s a great way to avoid waste.Makes one 11-inch crust (with extra left over), two 8-inch crusts, or 48 mini crusts.