In a food processor, (or a large bowl by hand), add the flour, baking powder, sugar, and salt. Pulse to combine.
With the food processor on low, drop in the cubes of butter, a little at a time. (If you are going by hand, cut in the butter with a pastry blender or rub between your fingers until it resembles a coarse meal.)
In a small bowl, whisk the egg, vinegar, and water.
With the food processor on low, gradually pour the mixture through the funnel until the dough just clumps together. Immediately turn the machine off.
Split the dough in half (unless you made the full recipe, then split in 4).
Wrap in cling-film and chill 1 hour.
Roll out the dough on a lightly floured surface. Make sure to turn and flip the dough. Reapply more flour as needed to stop the dough from sticking.
Using a cookie cutter, biscuit cutter, or an upside down cup a little bigger than the muffin cups, cut out circles.
Collect the dough back together, roll, and cut until all the dough is used up.
Press the dough in to the muffin cups, smoothing out the dough and forming it to the inside of the tin.
Chill 1 hour.
Preheat the oven to 325 ºF (160 ºC)
Pierce holes with a fork, in the bottom of the tart shells
Place a muffin liner in the inside of each shell. Place some dried beans, rice, or pie weights in each one.
Bake for 15 mins.
Remove the muffin liners and weights. Bake another 8-10 mins or until lightly golden.
Let cool. They are ready to fill!