Slice and Bake Vanilla Sugar Cookies from Cook's Illustrated

1/3 C granulated sugar
2 T brown sugar
1/2 tsp. salt
12 T unsalted butter, softened and cut into pieces
2 tsp. vanilla extract
1 large egg yolk
1 1/2 C all purpose flour
Process sugars and salt in food processor until no lumps of brown sugar remain, about 30 seconds. Scrape down sides of food processor, add flour, and pulse until a dough forms. Turn dough onto lightly floured surface and roll into a 10"log. refrigerate until firm 2-3 hours or up to 3 days. Heat oven to 350 degrees F. Line a baking sheet with a Silpat pad Slice chilled dough into 1/4" rounds and place 1" apart on baking sheet. Bake for about 15 minutes or until the edges are golden. Let cool 10 minutes on sheets and remove to wire racks.