If your confectioners sugar has turned lumpy over time, do not discard it. Pour the lumpy sugar into a blender container and process on medium speed until it becomes smooth enough to measure.
1 cup butter
1/2 cup confectioners sugar
1 3/4 cup all-purpose flour
1/4 tsp salt
1/2 tsp vanilla extract
2 Tbsp sugar
Preheat oven to 350°F. Beat butter in a large bowl with an electric mixer set at medium speed until fluffy, about 1 minute. Add confectioners sugar; beat until thoroughly combined.
Mix flour and salt in a medium bowl. Gradually beat flour mixture into butter mixture; stir in vanilla.
Press dough into an 8 x 8-inch pan. Bake until lightly browned around edges, about 20 minutes.
Sprinkle shortbread with sugar. Let cool in pan before cutting into small bars.
Before baking, score the dough into bars with a knife. The bars will be easier to cut and are less likely to crumble.