Chocolate Toffee Diamonds

Preheat oven to 350°F. Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes in a standing mixer (preferably fitted with paddle attachment) or 6 minutes with a handheld. Beat in yolk and vanilla, then add flour and salt and mix at low speed until just combined. Spread batter evenly with offset spatula in a greased 15 1/2- by 10 1/2- by 1-inch baking pan and bake in middle of oven until top puffs slightly and is firm to the touch and edges pull away from sides of pan, 20 to 25 minutes (watch carefully toward end of baking; cookie base can burn easily). While cookie base is baking, melt chocolate in a double boiler or a metal bowl set over a saucepan of barely simmering water, stirring occasionally, until smooth, then remove top of double boiler or bowl from heat. Spread chocolate over warm cookie base and immediately sprinkle with walnuts. Cool completely in pan on a rack, then cut into 2-inch diamonds or squares. Chill in pan until chocolate is firm, about 15 minutes.