Preheat oven to 350˚F (180˚C).
Cream butter and brown sugar until light and fluffy; add egg and vanilla and beat well.
Combine flour, baking powder and salt and gradually add to creamed mixture, mixing well after each addition.
Refrigerate dough for 1/2 hour. Roll into 1 inch (2.5 cm) balls and place 2 inches (5 cm) apart on greased baking sheet.
With your little finger or the end of a wooden spoon handle, make a depression in each ball.
Combine filling ingredients and spoon about 1 teaspoon into each cookie.
Bake for 12 – 15 minutes or until lightly browned. Cool for 5 minutes before removing to cooling rack.