In a large bowl, cream 9 tablespoons unsalted butter with a spatula or wooden spoon until it is smooth.
Add ¾ cup powdered sugar and mix until all is incorporated.
Add 1 egg and stir with a fork. It will not be smooth or completely incorporated. That is fine. Add half of the flour (eyeball it) and a pinch of salt.
Mix it but don’t wait until the flour is completely incorporated before adding the second half. Then, mix it until lumpy and a few dry spots remain.
Turn it onto a clean surface or counter and mix it with your hands, making it come together but touching it as little as possible. Don't overmix it. It should not be kneaded, only gathered into a disc or ball.
When you have a smooth dough ball, wrap it in plastic and refrigerate for at least 1 hour and up to 2 days.
Use an 8-inch square pan or 9-inch round pan.
When ready to roll, take the dough from the fridge and let barely soften at room temperature. The dough must still be cold. Simply wait until you can start to roll it, but not until it is very soft. This is important to ensure a tender dough.
On a lightly floured counter, start rolling the dough disc, making sure you do quarter turns (I do them to the left) often to ensure the dough is not sticking and adding more flour to the counter if it is sticking.
Once you have the dough rolled a few inches larger than the pan, fold it in half and place the fold in the middle of the pan.
Open the dough and line the pan, being careful not to stretch the dough. Remove excess dough by rolling over the rim of the pan with the rolling pin.
Prick the bottom with a fork, freeze for 20 minutes, or put it in the refrigerator for 1 hour.
Preheat the oven to 350ºF (180ºC).
Line the tart pan with a piece of aluminum foil and add beans or pie weights, covering the bottom evenly.
Bake the tart shell for 15 minutes. Remove the foil carefully and continue baking for 10 to 15 minutes more until the dough is fully baked.
Let cool completely on a wire rack. Remove from the pan and transfer to the serving plate.