Preheat oven to 350°. Line bottoms of 3 greased 8-in. round baking pans with parchment; grease parchment. In a large bowl, cream butter and brown sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in melted chocolate and vanilla. Combine flour, baking soda and salt; add to creamed mixture alternately with sour cream, beating well after each addition. Gradually beat in water.
Pour into prepared pans. Bake until a toothpick inserted in the center comes out clean, 40-45 minutes. Cool for 10 minutes before carefully removing from pans to wire racks to cool completely.
For frosting, in a large bowl, beat butter until smooth. Gradually beat in confectioners’ sugar, vanilla, salt and enough milk to reach desired consistency; remove 1-1/2 cups frosting and set aside. Tint remaining frosting with green gel food colouring until desired color is reached. Tint reserved frosting with black food colouring.
If cake layers have rounded tops, trim with a serrated knife to make level. Place 1 cake layer on a serving plate; spread with 2/3 cup frosting. Repeat layers. Top with remaining cake layer. Frost top and sides of cake.
For decoration, place melted candy coating in piping bag fitted with a small round tip; pipe witch hat shapes onto waxed paper. Let stand until set. Top cake with assorted sprinkles. Place black frosting in piping bag fitted with large star tip; pipe onto top of cake. Just before serving, gently stand 1 witch hat on top of cake. Adhere remaining witch hats and black pearls to sides of cake by gently pressing into frosting.