Combine in small bowl; whisk to blend:
1/2 cup all purpose flour
1 teaspoon baking powder
1/4 teaspoon salt
Stir chocolate and butter in top of double boiler set over simmering water until melted and smooth. Remove from over water. Cool mixture to lukewarm.
1 pound bittersweet (not unsweetened) or semisweet chocolate, chopped
1/4 cup (1/2 stick) unsalted butter
Using electric mixer, beat in bowl until thick, about 5 minutes:
1 ¾ cups (packed) brown sugar
4 large eggs
Beat in chocolate mixture and:
1 T. vanilla extract
Stir in flour mixture, then stir in:
5 1.4-ounce chocolate-covered English toffee bars (such as Heath),
1 cup walnuts, toasted, chopped
Chill batter until firm, about 45 minutes.
Preheat oven to 350°F. Line 2 large baking sheets with parchment or waxed paper. Drop batter by 1/4 cupfuls onto sheets, spacing 2 1/2 inches apart. Bake just until tops are dry and cracked but cookies are still soft to touch, about 15 minutes. Cool on sheets. (Can be made 2 days ahead. Store airtight at room temperature.)
Makes about 18.