Preheat oven: 350° F.
Cream butter and sugar together until combined well. Do not over-beat because you don't want to beat in too much air.
Add egg yolk and vanilla to the mixture and beat until combined fully.
Finally, add in the flour, sprinkle the salt on top of the flour, and mix together until fully combined.
Cover and refrigerate for 20-30 minutes to let the flour absorb into the mixture. Then check the dough. If it is still a bit sticky, add a small amount of flour.
After stamping, bake for 10-15 minutes until desired browning. Cool on wire rack.