Step 1 In the bowl of a stand mixer, or in a large bowl using an electric mixer, mix together the flour and powdered sugar. With the mixer running, start adding the butter cubes and continue mixing until all the butter is added and the mixture has a wet sand-like consistency.
Step 2 Make an indentation in the middle of mixture and add the egg yolk and water. Mix just until all of the ingredients are combined to form a dough. Form the dough into a flat square and wrap tightly in plastic wrap. Chill until the dough is firm, at least 30 minutes.
Step 3Remove the dough to a lightly-floured work surface and roll to a thickness of about 1/8-inch. Use a 3-inch round cutter to cut 20 circles from the dough. Arrange the circles on 2 parchment-lined baking sheets and refrigerate for at least an hour.