To prepare the crust, combine the butter, flour, baking powder and salt in a food processor. Process until the dough starts to resemble a coarse sand. You may also do that with your fingertips.
Take everything out of the food processor and place in a large bowl. Make a well in the center and break the egg. Sprinkle the sugar and mix again using your fingertips. Be careful not to overmix.
Bring the dough together by flattening it with your hands. Roll it into a ball. Cover with cling film and take it to the fridge for half an hour.
Meanwhile, preheat the oven to 350F. Butter and flour a 9’’ tart pan. It would be recommended to have removable bottom. The other option you can go for consists of 6 4’’ tartlet molds.
Take the dough out of the fridge. Use a rolling pin and flatten the dough without previously removing the cling film. After you get a large disk, about ¼ ‘’ thick, remove the foil. Transfer the dough to the pan. Use a fork to prick the bottom of the pastry shell.
If you prepare tartlets, lightly flour your work surface. Divide the dough into six equal parts. Work with only one piece at a time. Sprinkle some flour on top. Use the rolling pin to flatten to a disk of ¼ ‘’ thick and about ½ ‘’ larger than the tartlet mold. Carefully take the dough and place it in the tartlet pan. Press it gently against the sides and bottom using your hands. Get a sharp knife and trim the ends that hang over the edge of the mold. Repeat this process with the remaining dough pieces. Prick holes on the bottom with a fork.
In both cases (for the big tart or tartlets), bake for about 20 minutes. Let it chill for a few minutes, and then transfer to a wire rack to cool completely.