Preheat oven to 325° F. Line two 12-cup muffin pans with liners.
Melt chocolate and butter in double boiler, stirring occasionally. Beat sugar and eggs in large mixer bowl on high for about 3 minutes or until mixture falls from beaters in ribbons. With mixer at medium speed, add melted chocolate mixture to egg mixture. Gradually add flour. Stir in nuts. Scoop 1/3 cup batter into each cupcake liner, filling 3/4 full.
Bake for 25 to 30 minutes or until toothpick inserted in centers comes out with loose crumbs attached. Cool in pans for 5 minutes; remove to wire racks to cool completely.