BROWNIES: Heat oven to 160ºC . Lightly grease an 20 cm square baking pan. Melt chocolate and butter in a small saucepan over low heat. Cool 5 minutes. Put sugar and chocolate mixture in a medium-size bowl. Beat with electric mixer until blended. Add vanilla. With mixer on medium-low speed, add eggs, one at a time, beating after each until blended. Scrape bowl with rubber spatula. On low speed beat in flour until blended. Stir in half cup nuts. Spread batter in prepared pan. Bake 35 minutes or until thin crust forms on top and tester inserted in center comes out with a moist crumb. (Center shouldn't rise as high as edges. Cool on wire rack before frosting. FROSTING: Melt chocolate in top of a double boiler or bowl placed over simmering water. Pour evaporated milk, sugar and melted chocolate into blender. Blend on medium-low speed 50 seconds or until thickened (sound of machine will change when this happens). Spread over cooled brownies. Frosting will remain soft, but will set and become more firm. Sprinkle with remaining walnuts. Let stand 1 hour before cutting in squares. NOTE1: Frosting recipe may be doubled for a thicker layer of frosting. NOTE2: Make a day ahead so flavor can develop!