Combine first 4 ingredients in a large mixing bowl; add chocolate and boiling water, stirring until chocolate melts. Stir in egg yolks and liqueur. Beat egg whites at high speed with mixer until foamy. Add 1/2 cup sugar, 1 T at a time, beating until stiff peaks form and sugar dissolves. Stir 1/3 of egg whites into chocolate mixture. fold in remaining egg whites. Pour into a lightly greased 8 inch spring form pan. Bake at 375 for 28 min or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 mins. remove sides of pan and cool completely. Cake will crack on top. Sprinkle with powdered sugar.
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