Preheat the oven to 350°F (180°C).
Place the flour, sugar, salt and baking powder in a large bowl. Stir together to combine.
Using a pastry cutter, cut the shortening into the flour mixture until it forms crumbs, about the size of peas.
Place the egg in a small bowl, whisk it lightly, and divide it into two portions.
Place one half of the whisked egg and the vinegar in a liquid measuring cup, and whisk them together until well combined. Then add ice-cold water to the measuring cup, so that the total volume of the half egg, vinegar and water equals ⅓ cup of liquid. The total amount of water will vary slightly depending on the size of your half egg.
Make a well in the flour and pour the liquid mixture into it. Use a pastry cutter to mix it all together gently, just until a solid dough is formed. Be careful not to overmix.
Form the dough into a ball and place it on a clean, lightly floured surface. Separate the dough into two portions and set half aside for the top crust. Use a rolling pin to roll out the other half of the dough to about 10 inches in diameter.
Fit the crust loosely into a 9-inch pie pan by gently pressing it down. There’s no need to grease the pan. Set it aside while you prepare the filling.