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).
Cook butter in a small heavy saucepan over medium heat, stirring constantly, 6 to 8 minutes or until butter begins to turn golden brown. Immediately remove from heat. Pour butter into a bowl. Cover and chill 1 hour or until butter is cool and begins to solidify.
Beat butter and cream cheese at medium speed with an electric mixer until creamy; gradually add powdered sugar, beating until light and fluffy. Stir in cinnamon and vanilla.
Pipe frosting onto cupcakes. Garnish, if desired.