Preheat oven to 350 degrees F.
In a large bowl, cream together the butter and sugar until smooth. Gradually stir in the flour and custard powder until well blended. Start with 2 cups butter. Add more if you can’t keep the dough together enough to roll it out. It takes some work and some getting used to. If your dough seems too sticky, add a bit more flour at a time until it can be rolled out.
On a lightly floured surface, roll the dough out to ¼ inch thickness. Cut into rounds using a cookie cutter.
Bake for 8 to 10 minutes. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.
Sandwich one teaspoon of preserves between two cookies. Repeat until all cookies are used, or all jam is gone, and remaining cookies must be sacrificed to the baker for all her hard work.
In a medium bowl, gradually stir the milk into the confectioners’ sugar until the icing is of a spreadable consistency. Spread on top of the sandwiched cookies. Top each cookie with half a cherry while the icing is till wet. If you’re feeling like a rebel, go ahead and top with something else. Anything you like would work just fine.
Here is a recipe for making your own custard powder to use:
1 cup confectioners' sugar;
1/2 cup powdered milk;
1/2 cup cornstarch;
1 teaspoon vanilla powder (if you don't use, add 1 teaspoon vanilla extract to shortbread recipe);
1 teaspoon salt;
Whisk together and then measure out 1 cup to use for shortbread recipe.
If you substitute flour for the custard powder, but sure to add 1 teaspoon of salt to the shortbread recipe.