In a large mixing bowl or bowl of a stand mixer, whip the butter until fluffy. Whip in eggs, followed by sugar and flour, mixing until dough just comes together.

Flatten the dough into discs. Wrap in clingfilm/plastic wrap and chill at least one hour, or overnight.

While dough chills, preheat oven to 350° F / 175° C.

With a wooden rolling pin, roll the chilled dough very, very thin (this is the key to a good cookie!) The dough should be almost (but not quite) translucent; it should not be so thin that it breaks or falls apart when you move the cookies.

Cut dough into desired shapes with a metal cookie cutter.

With a metal spatula, transfer cookies to a large unlined baking tray.

With a pastry brush, brush the tops of each cookie with a very thin layer of egg wash. Sprinkle with desired toppings.

Arrange on an unlined tray and bake in preheated oven for 8-10 minutes, or until very lightly brown around the edges. (Keep an eye on the oven—sand tarts burn easily!)

Transfer cookies to a wire rack to cool. Store in an airtight container at room temperature for up to two weeks.