Preheat the oven to 350 degrees. In a large bowl, cream the butter and sugar and stir in the vanilla extract. In a medium-sized bowl, mix the flour and salt. Combine the flour mixture and the butter mixture until the dough is crumbly but holds together when pinched. Take a walnut-sized piece of dough in one hand. With the thumb of the other hand, gently knead it three times. Roll it into a ball and place on an ungreased cookie sheet. With your knuckle or thumb, press a well in the center and fill with 1/2 teaspoon of jelly. Bake for 10 to 15 minutes. Makes about 6 dozen cookies.