In a large bowl, beat butter and confectioners' sugar on medium speed until light and fluffy, 2-3 minutes. Beat in vanilla extract.

In a small bowl, whisk together flour and salt. Gradually add flour mixture to butter mixture, beating on low speed, until incorporated, 1-2 minutes.

Transfer dough to a greased square 9-in. baking dish. Use a spatula to flatten dough in one layer. Cover; chill at least 30 minutes.

Preheat oven to 300°. Use a knife to score batter into 2-in. squares. Use a fork to prick a pattern onto the top of each rectangle. Bake 40-45 minutes or until golden brown and firm in the center. While warm, use a sharp knife to cut 2-in. squares. Let cool completely before serving.