If using cookie cutters of varying sizes, bake like-sized cookies together. Smaller cookies will bake faster than larger ones.
1 cup icing sugar
2-1/4 cups unsalted butter, room temperature
4 cups all-purpose flour
1 egg white
2 tablespoon water
Decorating sugar/sprinkles (as much as desired for decorating)
Preheat oven to 325°F
Sift icing sugar.
Combine soft, room-temperature butter and icing sugar in the bowl of a stand mixer, and beat together with a paddle attachment on low speed until incorporated.
Turn mixer up to medium speed and beat until butter/sugar mixture becomes pale and fluffy (it will have tripled in volume).
Sift in the flour and mix on low speed until just combined. Be sure not to overmix.
Immediately spoon out 1/4 of the mixture and roll between two pieces of parchment/silicone paper to 1/4 inch thickness.
Repeat step #5 with remaining 3/4 of dough.
Refrigerate rolled sheets of cookie dough for a minimum of 1 hour, until hard enough to cut.
Peel off top layer of parchment/silicone paper, and cut out cookies using cookie cutters of desired shapes and sizes. Cut as close together as possible to reduce the quantity of scrap dough.
Gently lift cut cookies and place onto parchment/silicone lined baking sheet, leaving 1 inch between cookies. (If cookie dough is beginning to soften and becomes difficult to handle during cutting, you may refrigerate dough for few minutes, and then continue).
Combine egg white and water, and whisk to incorporate.
Gently brush a very light layer of egg white/water mixture over the top of each cookie, and then sprinkle with decorating sugar/sprinkles. This mixture will help the decorating sugar/sprinkles adhere to the cookie.
Bake on middle rack at 325°F 15 minutes. Cookies should have very little or no colour at all.
Cool before handling.