Preheat oven to 325°F.
Line cookie sheets with parchment.
Sift together the flour and salt and set aside.
Cream the butter and powdered sugar together until light and fluffy.
Mix in the evaporated milk and vanilla.
Mix in dry ingredients until just combined; do not overmix.
Split the dough in half, shape each into a small rectangle, and chill the dough for 20-30 minutes for easier handling.
Roll out the dough, half at a time, on a floured surface into a 12x8 inch rectangle (roughly); trim sides.
Cut into 2x2 inch squares and place on the prepared cookie sheets.
Bake until firm to the touch and very lightly golden, about 12-15 minutes (check at the 10-12 minute mark to see how things are coming along).
Transfer to a wire rack and allow to cool fully before topping.
These can be made ahead and stored in an airtight container on the counter until you’re ready to top.