Process the butter and brown sugar in a Manual Food Processor until creamy. Add egg yolk and vanilla; process until combined.
Add salt and flour; process until soft dough forms.
Using Small Scoop, drop 18 level scoops of dough onto small circles on baking mat. Press the back of scoop into the dough to make wells, dipping back of scoop in sugar to prevent sticking. Bake 9-11 minutes (15-17 minutes for Stoneware) or until the edges begin to brown.