1. Heat oven to 350 degrees F. Lightly grease and flour two 9-inch round cake pans. Sift flour, cocoa, baking soda and salt together in bowl. Set aside.
2. Beat the 1-1/2 cups butter and granulated sugar in large bowl at medium-high speed of electric mixer until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add 1 teaspoon vanilla. With mixer at low speed, add dry ingredients alternately with buttermilk, beginning and ending with dry ingredients, until blended. Spread batter in prepared pans; bake 40 minutes or until toothpick inserted in center of cakes comes out clean. Cool in pans on wire rack 20 minutes. Run small knife around edge of pans. Invert and remove pans; cool completely.
3. For the Chocolate-Cream Cheese Icing, beat the 3/4 cup butter and cream cheese in large bowl on medium speed of electric mixer until smooth. Beat in confectioners' sugar. Gradually beat in cooled unsweetened chocolate and 1 teaspoon vanilla; increase speed to medium-high and beat 1 to 2 minutes more or until light and fluffy.
4. Place one cake layer on plate and spread with 1 cup Chocolate-Cream Cheese Icing. Top with second layer and spread sides and top with remaining icing.
5. Melt white chocolate in a small microwave proof bowl on 50 percent power (Medium) 30 seconds; stir until smooth. Transfer to a heavy-duty resealable plastic bag. Snip off one corner and pipe spider web over top and side of cake. Melt semisweet chocolate in another small microwave proof bowl on 50 percent power (Medium) 30 seconds; stir until smooth. Transfer to another heavy-duty resealable plastic bag; cool 2 minutes. Place peppermint patty on top of cake. Pipe 2 white chocolate dots for eyes, then 2 semisweet chocolate dots for pupils and pipe 8 legs. Makes 12 servings.