Poppy Seed Thumbprints

0 Preheat oven to 350F.
1 In a large bowl sift together the flour and the salt. Whisk in the poppy seeds and set aside.
2 In a large bowl of an electric mixer fitted with the paddle attachment, mix the butter until smooth. Pour in the sugar and mix until well incorporated. Add the egg yolks and vanilla. Mix well. 
3 With the electric mixer speed at low, add the dry ingredients until well combined together.  Do not overmix.
4 Roll the dough into small balls and place them 2 inches apart on a parchment line cookie sheet. Using a wooden spoon with a rounded handle no wider than ½ inch, make a deep indentation with the tip of the handle in the center of the cookie.
7 Place the cookies in the oven and bake for 10 minutes.  Remove from the oven and re-press each indentation if needed it. Then fill the center with the raspberry preserve using a small spoon. Do not overfill it as it will run over.
6 Return the cookies to the oven and bake for another 5 minutes or until the cookies are golden brown around the edges. Cool on a wire rack.