Beat the butter, sugar, salt and vanilla together until smooth and creamy

Mix in the egg yoke until well incorporated, scraping down the sides of the bowl at least once

Scrap onto a lightly floured board and knead a few times, just until the dough smooths out

Add the flour and mix just until incorporated

Turn onto a sheet of plastic wrap and roll into a log, wrap up and refrigerate for several hours or freeze

Before baking, preheat the oven to 163 c

Line your baking sheets with parchment

Slice the dough into slices about 1/8" thick and place them on the sheets about an inch apart

Bake until just beginning to turn golden around the edges, about 16-18 minutes