Preheat oven to 350 degrees F. In a large bowl, combine butter and sugar. Beat with an electric mixer on medium speed until combined. Add egg yolks and liqueur. Beat until combined, scraping side of bowl occasionally. In a small bowl, combine flour, poppy seeds, orange peel, and salt. Gradually add the flour mixture to the butter mixture, beating on low speed just until combined.

Shape dough into 1-inch balls. Place balls 1 inch apart on an ungreased cookie sheet. Lightly press the tip of your thumb into the center of each ball of dough.

Bake in the preheated oven for 10 to 12 minutes or until edges are lightly browned. If an indentation puffs up during baking, re-press your thumb (or if cookies are too hot, use the bowl of a measuring teaspoon) into each puffed-up cookie.

Transfer cookies to a wire rack; let cool. Fill the center of each cookie with about 1/2 teaspoon of the jelly. Makes about 36 cookies.

To Store: Layer filled cookies between waxed paper in an airtight container; cover. Store in the refrigerator for up to 3 days or freeze for up to 1 month.