Snowflake Sugar Cookies

Combine the flours and baking powder in a bowl; stir to evenly distribute the baking powder. Set aside.

In a 6-quart stand mixer with a paddle attachment, cream the butter and sugars until light and fluffy. Add the salt, vanilla and milk; mix to blend. With mixer running, add eggs and extra yolk, one at a time, waiting until each is combined before adding the next.

Reduce the mixer speed to low; slowly add flour mixture. Add additional all-purpose flour 1 tablespoon at a time until the dough is no longer sticky to the touch. Turn dough out onto plastic wrap; divide in half. Wrap each portion in plastic wrap; form into flat discs. Refrigerate 4 to 48 hours, or freeze up to a month.

Heat oven to 375 degrees F. Let dough warm up slightly; roll out to 1/2 inch thick using powdered sugar (not flour) to stop it from sticking. Cut out snowflake shapes; bake on parchment-lined baking pans until just barely starting to brown on the edges, 12 to 15 minutes. Remove promptly from oven; let cool on the sheet pan, 2 minutes. Transfer to a wire rack to cool completely before decorating.

For icing, mix the meringue powder and powdered sugar in a medium bowl; slowly add water until the frosting reaches desired consistency. Add a little gel food coloring, if desired. For best results, start with a stiff frosting, put some aside in a parchment baking bag or zip-top bag, then add water to the remaining frosting. Use the stiff frosting to pipe outlines on the cooled cookies, then carefully apply the thinner icing to fill in the cookies between the piped outlines (a toothpick helps to carefully move thin icing into small spaces.