01 -
Beat the softened butter with the sugar for 2-3 minutes until pale and fluffy.
02 -
Mix in the egg yolks one at a time, then the vanilla, scraping down the bowl.
03 -
Add the flour and salt in two additions, mixing just until a soft dough forms. Do not overmix.
04 -
Wrap the dough and refrigerate for at least 30 minutes, until firm but pliable.
05 -
Roll into 1-inch balls. If using the walnut coating, roll each ball in whisked egg white, then chopped walnuts.
06 -
Press a deep thumbprint into the center of each ball and spoon in about 1/2 teaspoon of jam.
07 -
Bake at 350°F (175°C) for 14-16 minutes, until the edges are pale gold. Cool on the sheet 5 minutes, then move to a rack.