In a large bowl sift together the flour, cornstarch, icing sugar, baking powder and salt, make a well in the centre and add the egg and butter combine with a fork until mixture is almost combined. (if you wish you can use a food processor or stand up mixer with the paddle attachment).
Move to a lightly floured flat surface and gently knead until you have a soft smooth dough (do not over knead) (if too sticky then add a little extra flour if too dry add a little more butter (a tablespoon at a time).
Wrap the dough in plastic wrap and refrigerate for 30 minutes.
Remove from the fridge and roll half the dough at a time on a floured flat surface until a little bit thicker than a pie crust (about ¼ inch), cut out with a medium (2¼ - 2½ inches / 7 cm) or small (2 inch /5 cm) round cookie cutter.
Place a teaspoon of jam in the middle of each circle and take the two ends and pinch them together, dip fingers in a little water and pinch the dough again, just to make sure that the dough sticks and doesn't open while baking.
Place the cookies on 1-2 parchment paper lined cookie sheets. Preheat oven to 350F (180C), while the oven is preheating refrigerate the cookies, then bake for approximately 12-15 minutes. Let cool completely then dust with a little icing / powdered sugar. Enjoy.
Notes
Be sure to use an all purpose flour with at least 11.5% protein, this will help keep the cookies from spreading. For Room Temperature remove ingredients from the fridge for approximately 60 minutes before using (longer if your house is cool).You can even bring the dough together with a food processor or stand up mixer with the paddle attachment.