In a large bowl, mix the plain flour and caster sugar evenly.Add the softened unsalted butter and gently rub it with your fingers until the mixture resembles breadcrumbs. Gradually add water, one tablespoon at a time, until the dough begins to come together. Be careful not to add too much water; just enough to make the dough hold together. Remove the dough and gently knead it into a ball on a clean surface.Wrap the dough in plastic wrap and place it in the refrigerator for 20 minutes to make it easier to handle.
Lightly sprinkle some flour on the chilled dough and use a rolling pin to roll it into thin pastry.Use a circular cutter or the bottom of a glass to cut the pastry into small circles.Place each small pastry circle into the cupcake tin or mini pie tin, pressing them firmly against the bottom and sides.Fill each pastry with a generous amount of mincemeat, being careful not to overfill.Brush a little egg wash along the edges of the pastry, then cover with another small pastry circle, pressing the edges together with your fingers to seal, ensuring the filling doesn't spill out.Use a fork to gently pierce a few small holes on the top of each pie to allow steam to escape.
Place all the mini mince pies in the preheated air fryer oven and bake at 180 degrees Celsius for about 15 minutes or until the pastry turns golden brown.Remove from the oven and let the mini mince pies cool slightly, then carefully remove them from the tins.