Raspberry Thumbprint Cookies

1. In a large bowl, cream together butter and sugar with a mixer until well blended. Add the vanilla, salt, and egg yolk, and beat until fluffy.

2. Add flour and mix until it forms a stiff dough. Scrape dough onto a large sheet of wax paper and roll it back and forth to make a long rope about 3/4 inch in diameter and 12 inches long. Roll it up tightly in the wax paper and refrigerate until firm, about 30 minutes. (The dough can be prepared to this point up to 3 days in advance.) Remove from the fridge and let it rest for 10 minutes at room temperature before proceeding.

3. Preheat oven to 300 degrees F. Unwrap and slice chilled dough into 1/2-inch-thick pieces. Roll them into ball shapes and space them 1 inch apart on an ungreased cookie sheet. Make a small indentation in the center of each one by pressing down on it with your thumb. Fill each hollow with 1/4 teaspoon of preserves.

4. Bake, rotating pan once, until very lightly browned, 18 to 20 minutes. Remove from the oven and allow to rest for at least 5 minutes on the baking sheet before transferring to wire racks to cool. Careful: the jam is very hot!