Keep in mind that these cookies are different from some other baking recipes in which you’re looking to cream the butter and the sugar for a number of minutes to introduce air into your dough. Here, you really just need to mash the sugar into the butter we
Preheat the oven to 375 F. Line a large baking sheet with parchment paper.
In a medium bowl, cream the butter and sugar together with a fork (or your hands). Add the egg yolks, vanilla extract, salt and flour. Mix well and knead together a few times with your hands to gather the crumbly pieces of the dough together.
Roll walnut-sized pieces of dough into a ball.
Whisk the egg whites until slightly foamy and place poppy seeds in a nice, shallow bowl.
Roll balls of dough in egg white and then in the poppy seeds. Set on baking sheet and flatten slightly with the palm of your hand. Make indent with your thumb and fill with 1/4 – 1/2 teaspoon of jam.
Bake for 18-20 minutes or until the dough has turned a golden brown — the cookies should feel soft and they will firm up as they cool. Cool on cookie sheet for 15 minutes. Enjoy warm. Or room temperature. In the morning — or in the evening. Store covered at room temperature.