In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg yolk and vanilla. Gradually add flour and salt, beating until smooth.
Press into a greased 15x10x1-in. baking pan. Bake at 350° for 17-19 minutes or until light golden brown. Immediately place chocolate bars on top; return to the oven for 1 minute. Spread melted chocolate over bars; sprinkle with pecans.