These are perfect to serve as an after-lunch cookie or with tea because they are so light.
6 tablespoons butter
1/3 cup granulated sugar
1 to 2 egg yolks
1/4 teaspoon finely grated lime or lemon zest
1 cup all-purpose flour
With an electric mixer, beat butter and sugar until light. Beat in egg yolk (2 yolks will make a slightly richer cookie) and lime or lemon zest. Stir in flour with a spoon or rubber spatula. Refrigerate dough.
Roll chilled dough into 1-inch balls. Place balls on greased baking sheet. Slightly flatten each ball with your thumb or the bottom of a glass. Bake in a preheated 375 degree F oven for 10 to 12 minutes, or until edges brown. Let cool on a rack.