Preheat oven to 350°. Microwave morsels in a microwave-safe bowl at HIGH 1 to 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals.
Beat butter and sugar at medium speed with an electric mixer until well blended (about 5 minutes). Add eggs, 1 at a time, beating just until blended after each addition. Add melted chocolate; beat until blended.
Sift together flour, baking soda, and salt. Gradually add to chocolate mixture alternately with sour cream, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Gradually add hot water in a slow, steady stream, beating at low speed just until blended. Stir in vanilla.
Place 36 paper baking cups in 3 (12-cup) muffin pans; spoon batter into cups, filling three-fourths full.
Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in centers comes out clean. Remove from pans to wire racks, and let cool completely (about 45 minutes).