Preheat oven to 350, line a 9x13 pan with parchment.

Beat butter and sugar on medium speed until light, 2 minutes. Beat in yolk, vanilla and salt. On low, gradually beat in flour until just mixed. (Will be stiff.)

Pat dough evenly into the bottom of the lined pan. Bake in center of oven until pale gold, about 20 minutes.

Remove from oven and scatter chocolate evenly over the top. Put back in oven for 1 minute. Use knife to spread chocolate.

Cool completely in the pan on a wire rack. Cut into small squares.