In a bowl, whisk together flour, sugar & salt. Add cold butter, vanilla & lemon zest. Cut into flour mixture with a pastry blender until dough starts to come together & form clumps. Divide 2/3 of pastry between 10 tart/muffin cups. Using fingertips, evenly press the dough into each cup. With remaining pastry, divide it into 10 balls & flatten each to form a top for each tart. Place pastry in refrigerator until filling is prepared.