Cream butter and sugar in large bowl. Blend in melted chocolate. Add eggs and vanilla; beat until light and fluffy.
Sift flour, baking soda and salt. Add to creamed mixture alternately with sour cream. Beat well after each addition, then beat for 1 minute on medium speed.
Spoon into cupcake paper-lined muffin pans. Fill cups 1/2 full. Bake about 20 minutes at 375 degrees or until toothpick inserted in center comes out clean.
Cool completely and frost with "Easiest Chocolate Frosting". Makes 30 cupcakes.