Heat the oven to 350°F. Lightly grease a 13x9-inch baking pan. With a wooden spoon or a mixer, cream the butter and brown sugar until smooth and no lumps remain. Add the egg yolk, vanilla, and salt; beat until well blended. Add the flour; mix until dough begins to come together (if you’re using an electric mixer, set it on low speed). Pat the dough into the pan. Bake until dough begins to pull away from the sides of the pan and keeps a slight indentation when pressed lightly, 26 to 28 minutes.
Meanwhile, melt the chocolate and the milk in a double boiler, stirring as little as possible to prevent separating. Pour the warm ganache over the warm baked cookie crust and spread it evenly. Sprinkle with the nuts and let cool completely until chocolate has set, about 4 hours. Cut into 1-1/2-inch squares.
nutrition information (per serving):