In two batches, prepare and bake cakes according to package directions, using two greased and floured 13-in. x 9-in. baking pans. Cool for 10 minutes; remove from pans to cool on wire racks.
In a large bowl, cream butter until fluffy; beat in the chocolate, vanilla, confectioners' sugar and enough milk to achieve spreading consistency. Center one cake on a 16-in. x 12-in. covered board; frost top. Top with remaining cake; frost top and sides of cake. With a metal spatula, smooth frosting to resemble boards.
For chest lid, insert 4-in. skewers equally spaced 6 in. into one long side of corrugated cardboard lid. Frost top of lid. Cut a small hole in the corner of a pastry or plastic bag; insert star tip #21. Pipe a shell border on edges of lid and for handles on side of chest.
Place one 7-1/2-in. skewer on each side of cake top, about 3-1/2 in. from back of chest. Position lid over cake; gently insert short skewers into cake about 1 in. from back of chest. Rest lid on long skewers.
Arrange candy in chest. Cut a small keyhole from a fruit roll-up; center on front of cake. Position strips of fruit roll-ups in front and back of chest. Yield: 14-16 servings.