Preheat the oven to 350 degrees F and line two 9-by-13-inch pans with parchment paper; coat with cooking spray. Prepare the cake mixes as directed; divide between the pans and bake as directed. Let cool, then unmold.
Stack the 2 cakes with chocolate frosting in between. Mark the middle with toothpicks as shown, then cut in half crosswise. Stack the 2 halves with frosting in between to make a 4-layer cake. Freeze 30 minutes.
Put the cake on cardboard with a short side facing you. Mark the middle with toothpicks as shown. To form the roof, shave off the top 2 layers of cake at an angle, starting 1/2 inch to the right and left of the toothpicks.
Crumble about 2 cups of the cake trimmings and mix with just enough chocolate frosting so it sticks together. Moisten your fingers, then gently mold the top of the cake to form the tip of the roof.
Cut the graham crackers by scoring with a small serrated knife, then snapping along the scored line. You will need 16 graham cracker squares, 4 large triangles and 2 small triangles to cover the cake.
Cover the cake completely with the remaining chocolate frosting. Gently press the graham crackers into the frosting to cover the whole cake except the angled roof.
Cut 1 whole graham cracker into 4 small rectangles for the chimney. Insert into the roof as shown, making sure the tops of the graham crackers are aligned.
Arrange the cereal on the roof in rows, starting from the bottom and working your way up; overlap the rows so they look like shingles.
Decorate the house with vanilla frosting, candies and sanding sugar. To make icicles, pipe a dot of frosting, then drag the pastry bag downward. Top the chimney with cotton candy. Dust with confectioners' sugar.