Melt chocolate and butterin a heavy saucepan over low heat. Beat eggs at med speed with elec mixer until frothy; gradually add sugar, beating until thick and pale (about 5 min). Add choc mixture, stirring until blended. Combine flourand baking powder; stir into chocolate mix. Stir in nuts. Flour hands, form dough into a 13" log. Place on a lightly greased baking sheet. Brush with egg white. Bake at 350F for 45 minutes; cool on wire rack. Cut log with a serrated knife crosswise into 24 (1/2") slices, and place on ungreased cookie sheet. Bake at 350F for 10 min each side. Remove to wire racks to cool.