Place flour, butter, sage, vanilla, and egg yolk in a large bowl and begin mixing with a fork until no "dry" flour remains.
Dump flour mixture onto a lightly floured surface and knead until all ingredients are thoroughly combined. If there are still little balls of butter showing, that is fine.
Form dough into a ball and wrap in plastic wrap. Chill for an hour.
When ready to make cookies, preheat oven to 375 degrees and remove dough from fridge.
Place dough ball on lightly floured surface and gently roll dough out to a large rectangle about 1/4" thick, rolling in only one direction.
Using a cookie cutter (or I used a juice glass), cut cookies from dough and place on an ungreased, non stick baking sheet.
Combine egg white and water and brush the top of each cookie with mixture. Sprinkle half of the cookies with white sugar and arrange pretty, flat sage leaves on the other half. Lightly brush over arranged sage leaves with more egg white mixture before baking.
Bake cookies 8 minutes. Allow to cool completely before serving.