Preheat oven to 350°F.
Lightly grease baking sheets or line with parchment.
In a large bowl, beat butter, sugar, lemon rind or extract, vanilla and salt until very light.
Beat in egg.
Add flour, stirring until well blended.
Roll into 1 inch balls and arrange on cookie sheets at least 2 inches apart.
Use fingertip or thumb to put indentation into center of each.
Bake for 12 to 14 minutes.
Let cookies cool about 5 minutes to firm up, then move to a rack.
When completely cool, fill indentation with jam, using 1/4 to 1/2 teaspoon per cookie.
*This recipe makes about 5 dozen cookies.*
**This recipe brought to you courtesy of King Arthur Flour.**