How to Make It
Preheat oven to 300°F. Beat butter in a medium bowl with an electric mixer on medium speed until creamy. Gradually add sugar, beating until light and fluffy, about 2 minutes. Add egg yolks, 1 at a time, beating well after each addition. Stir in cherry liqueur. Add flour and salt; beat on low speed just until blended. Chill dough at least 1 hour.
Shape dough into 1-inch balls; place balls 2 inches apart on baking sheets lined with parchment paper. Press thumb into each ball to make an indentation.
Bake in preheated oven until very lightly browned around bottom edges, 18 to 23 minutes. Remove from oven, and lightly press thumb into indentations. If using chocolate kisses, immediately place 1 kiss in each indentation. If using jelly, cool cookies on baking sheets on wire racks, about 20 minutes, and spoon about 1/2 teaspoon jelly into each indentation. Cool cookies completely.