1/2 cup all purpose flour
1 teaspoon baking powder
1/4 teaspoon salt
12 ounces bittersweet (not unsweetened) chocolate, chopped
4 ounces semi-sweet chocolate, chopped
1/4 cup (1/2 stick) unsalted butter
1-3/4 cups (packed) brown sugar
4 large eggs
1 tablespoon vanilla extract
5 1.4-ounce chocolate-covered English toffee bars (such as Heath), coarsely chopped
1 cup walnuts, toasted, chopped
Combine flour, baking powder and salt in small bowl; whisk to blend. 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.
Using electric mixer, beat sugar and eggs in bowl until thick, about 5 minutes.
Beat in chocolate mixture and vanilla. Stir in flour mixture, then toffee and nuts. Chill batter overnight.
Preheat oven to 350°F.
Line 2 large baking sheets with parchment or waxed paper; spray Generoulsy with cooking spray. 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.)