Preheat oven to 350°.
Cream butter; gradually add brown sugar and beat well at the medium speed of an electric mixer.
Add egg yolk and vanilla; beat well.
Combine flour and salt; gradually add to creamed mixture, mixing well.
Spread batter in a lightly greased 13" x 9" x 2" pan.
Bake at 350° for 25 to 30 minutes (crust will be soft).
Remove pan from oven, and immediately arrange pieces of chocolate on the hot crust.
Let stand until chocolate softens; then spread evenly over curst with a spatula. Let cool slightly.
Cut into squares while warm. Let squares cool completely in pan.