Beat butter until creamed. Add brown sugar and beat until fluffy. Stir in vanilla and egg yolk until well-combined. Add flour and salt gradually. Chill dough for 30 minutes. Preheat oven to 350°F/180°C and line cookie sheets with parchment paper. Roll cookie dough into approximately 1-inch/2.5-cm balls and use thumb to indent center. Space cookies at least 1-inch/2.5-cm apart. Bake cookies for 10-12 minutes and cool completely on pan.
in small saucepan over medium-low heat, combine white chocolate chips and condensed milk. Stir frequently until chocolate is melted and ingredients are well-combined. Mix in lime extract. Spoon filling into thumbprints of cookies; sprinkles are optional. Allow to cool completely before serving.