In a large bowl sift together the flour and the salt. Whisk in the poppy seeds and set aside.
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.
With the electric mixer speed at low, add the dry ingredients until well combined together. Do not overmix.
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.
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.
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.
Storage Once the cookies have cooled to room temperature, store them in an airtight glass jar layered between pieces of parchment paper. They will last for up to 1 week.Make-Ahead - You can make the dough ahead of time and refrigerate for up to 24 hours before forming and baking the cookies. Refrigerator - While these cookies can be stored at room temperature, you can also store them in the fridge for up to a week in an airtight container. You can also wrap them individually in plastic wrap.Freezer: Freeze cookies for up to 3 months.