Mix in butter and rub butter with fingers into flour until mixture is crumbly.
Stir in the egg yolk(s) and vanilla.
Form dough into two logs about 2 inches in diameter. If dough does not come together, add water one tablespoon at a time until it just forms a ball. Wrap in plastic wrap and chill for at least one hour.
Cut the logs into slices about 1/4-inch thick. Optionally, stamp each slice with a cookie stamp.
Place slices on a parchment-lined baking sheet and bake until very lightly browned around the edges, about 8 to 10 minutes. Remove pan from the oven and allow cookies to cool for about 5 minutes.
Remove to wire rack and cool completely.
Roll the logs in sugar before slicing to get a crispy, sweet outer rim.
There are many variations of this recipe, including blue cheese and ginger/cardamom flavors. Check my blog post to see how to do them.