Position rack in center of oven and preheat to 160ºC F. Grease and flour 23 cm square baking pan. STEEL KNIFE: Combine toffee and walnuts in work bowl and chop coarsely using 6 to 8 on/off turns. Remove from work bowl and set aside. Combine sugar and chocolate in work bowl and mix using 6 on/off turns, then process until chocolate is as fine as sugar, about 1 minute. Add butter and blend 1 minute. Add eggs, vanilla and salt and blend until fluffy, about 40 seconds, stopping as necessary to scrape down sides of work bowl. Add flour and toffee mixture and blend using 4 to 5 on/off turns, just until flour is incorporated; do not overprocess (remove Steel Knife and blend mixture gently with spatula if necessary to mix in flour completely). Turn batter into prepared pan, spreading evenly. Bake until tester inserted in center comes out almost clean, about 50 minutes (for firmer, cakelike brownies, bake about 5 minutes longer). Let cool in pan on rack. Cut into 4 cm squares. Store in airtight container. These are dark, moist and crunchy with bits of toffee candy.