Place the flour, salt and baking powder in the jar of the food processor or large bowl if making it by hand. Combine all dry ingredients.
Scatter cubes of cold butter and process or use your hands or pastry cutter to integrate until the butter is the size of peas and beans. It will be irregular. Don't overprocess. Use short spurts and a spatula or spoon once to move the mixture so it doesn't pool at the bottom.
Lightly mix the egg with 2 tablespoons of water in a small bowl and add to the flour mixture. Process for a few seconds at a time or mix with a fork until the dough barely comes together. Don't mix or knead until you have a smooth ball at this point.
Transfer the buttery shaggy dough into a large mixing bowl or a lightly floured counter.
Add more cold water, a tablespoon at a time, until you have a dough that comes together. The key is to create a smooth ball of dough, touching it as little as possible so you don't warm the butter.
Wrap the dough disc and refrigerate for 1 hour. At this point, you can freeze it for a month or leave it in the fridge for 5 days, very well wrapped.
Roll out the dough on a lightly floured surface into a round no more than ¼ inch thick. If more convenient, you can roll the dough between two large sheets of parchment.
Preheat the oven to 350°F (180°C).
Line an 8-inch tart pan with removable bottom with the dough, letting it down without stretching it. Remove excess dough from the sides and prick the bottom with a fork.
Place a large piece of aluminum or parchment paper and top with baking weight, beans, rice or flour. It should fill the pan.
Bake for 15 minutes, until you can easily and carefully remove the paper with the weight. Bake for 5 more minutes until it starts to dry.
Let cool for 15 minutes before filling.