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 until firm, about 45 minutes.

Preheat oven to 350F. 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.)