Heat oven to 350°F. Grease three 8-inch square pans. Line pans with waxed paper; then grease and dust with flour. In medium bowl, combine flour, baking powder, and salt.
In bowl, beat sugar and butter for 2 minutes, or until light and fluffy. Add eggs, one at a time, beating after each. Alternate flour mixture and milk, beginning and ending with flour, beating until smooth. Stir in vanilla and orange rind.
Spoon half of the batter into a 1-quart resealable bag. Stir melted chocolate into remaining batter until blended. Spoon chocolate batter into a resealable bag. Snip a 1/2-inch corner from each bag. In two of the cake pans, pipe a 1-1/2-inch-wide strip of vanilla batter around the outer edge of the pans, then a 1-1/2-inch-wide strip of chocolate batter; fill in center with vanilla batter. For the third pan, pipe a 1-1/2-inch-wide strip of chocolate batter at outer edge, and follow with a 1-1/2-inch-wide strip of vanilla batter; fill in center with chocolate batter.
Bake 20 to 25 minutes, or until golden. Transfer cakes in pans to wire rack; cool 10 minutes. Invert onto wire rack and cool completely.
In large bowl, beat butter and sugar until fluffy. Add melted chocolate and sour cream. Gradually beat in the milk until smooth. Spoon one cup of the frosting into a pastry bag fitted with a medium star tip.
To assemble cake, place one vanilla-centered layer on cake plate; spread with frosting. Top with the reverse-design cake layer; spread with frosting. Top with remaining cake layer. Spread remaining frosting over sides and top of cake. Drag fork tines through frosting along sides of cake to add texture.
For candy decorations, working with several Tootsie Rolls at a time, heat in microwave for 4 to 5 seconds to soften. Roll out on a clean work surface to a scant 1/8-inch thickness. Cut into eight 1-3/4-inch squares. Repeat with Airhead candy. Place on top of cake in a checkerboard pattern. Roll and cut the remaining candies into 1-inch disks about 1/4 inch thick. Place on top of cake as game pieces. Using pastry bag with star tip, pipe a decorative edge around base and top of cake.