Using an electric mixer, beat butter and powdered sugar in a large bowl until light and creamy; add vanilla and beat to blend. Place flour, cornstarch and salt in small mixing bowl and mix with wire whisk; stir in walnuts. Add flour/nut mixture to butter mixture and mix just until blended. Refrigerate dough at least 1 hour.
Preheat oven to 325°F. Prepare baking sheets by lightly greasing with butter, or spray with non-stick vegetable spray. Drop cookie dough in rounded teaspoons one inch apart onto prepared pans. Press down center of each ball with thumb. Bake until light golden brown, 18 to 20 minutes. Cool completely. To serve, fill indentation with Lemon Curd. Makes about 65 Thumbprint cookies.
In the top of a double boiler, beat eggs and sugar. Stir in lemon juice, butter and lemon peel. Cook over simmering water for 15 minutes or until thickened. Immediately cover the lemon curd with plastic wrap to prevent a skin from forming and refrigerate. The lemon curd will continue to thicken as it cools. It will keep in the refrigerator for about a week. Makes about 1 cup.
Tip: To make the lemon curd lighter in texture and flavor, fold in a little whipped heavy cream once the curd has been thoroughly chilled.