Directions

• Step 1
  Preheat the oven to 350°F. Line the bottom of a 10" springform pan with parchment paper.
• Step 2
  Melt the chocolate, butter, and coffee in the top of a double boiler over hot, not simmering, water, stirring continuously. Stir in ½ cup cocoa powder. Set aside to cool to room temperature.
• Step 3
  Whisk together the egg yolks and ½ cup of the sugar until thick. Add the butterscotch liqueur, chocolate mixture, ground walnuts, and flour. Set aside.
• Step 4
  Whip together the egg whites and cream of tartar, using an electric mixer on a high speed, until soft peaks form. Sprinkle in the remaining sugar, beating until stiff peaks form. Gently fold a third of the egg whites into the chocolate mixture. Repeat until all the egg whites are incorporated.
• Step 5
  Pour into the prepared pan and bake for 35 minutes, or until a toothpick comes out with moist crumbs when inserted in the center of the cake. Cool in the pan on a wire rack. The center will fall slightly as it cools. Before serving, dust with remaining ¼ cup cocoa powder.