Preheat oven to 350 F
Beat the sugar and the butter in a mixing bowl until light and fluffy. Beat in the egg yolks and vanilla, then stir in the flour and salt to make a soft dough.
Spread the dough in a greased 13 x 9 x 2 inch baking pan. Level the surface. Bake for 25 - 30 minutes, until lightly browned. The texture will be soft.
Remove from the oven and immediately place the chocolate squares on the hot base. Set aside until the chocolate softens, then spread it evenly with a spatula. Sprinkle with the chopped nuts.
While it's still warm, cut it into 2 x 1 1/2 inch bars, remove from the pan and let cool on a wire rack.