Step 1Combine the flour, baking powder and salt in a small bowl; whisk to blend.
Step 2Stir the chocolate and butter in the top of a double boiler set over simmering water until melted and smooth. Remove the mixture from the heat and cool to lukewarm. Using an electric mixer, beat the sugar and eggs in a large bowl until thick, about 5 minutes. Beat in the chocolate mixture and the vanilla. Stir in the flour mixture, then the toffee and nuts. Chill the batter until firm, at least 45 minutes and up to one day.
Step 3Heat the oven to 350 degrees. Line two large, rimmed baking sheets with parchment paper. Drop the batter by one-quarter-cupfuls onto the baking sheets, spacing them about 2 1/2 inches apart. Bake the cookies just until dry and cracked on top but still soft to the touch in the center, about 15 minutes. Cool on the baking sheets. (The cookies can be prepared two days ahead; store in an airtight container at room temperature.)