Preheat oven to 350°F. Line a 9x13-inch baking pan with wax paper. Grease and flour paper.
In a mixer, beat cake mix, buttermilk, eggs, and oil in a large bowl until smooth, about 2 minutes. Pour batter into prepared pan and bake until golden and a toothpick inserted into center comes out clean, about 35 minutes. Transfer to a wire rack and cool for 10 minutes. Invert and cool completely.
Roll out white spice drops to a 1/8-inch thickness in the blue decorating sugar, keeping the candy covered in the sugar to prevent sticking. Repeat with green spice drops in granulated sugar, yellow spice drops in yellow sugar, and orange spice drops in orange sugar. Use a small flower-shaped cookie cutter or scissors to cut out desired number of flowers and leaves from flattened spice drops.
Microwave Tootsie Rolls on high for 3 seconds to soften. Press them together; roll out. Use a round cookie cutter to cut out a 2-inch circle.
Spoon 3 tablespoons chocolate frosting into a zip-top bag; set aside. Gently mix 1/2 cup chocolate frosting with the vanilla frosting. Leave some chocolate streaks to create wood effect.
Place the cake on a flat work surface. Trim 1/2-inch from a short end of the cake. Cut two corners from the opposite short end, measuring 3-1/2-inches. Frost the top and sides of the cake.
Snip a small corner from the bag of chocolate frosting. Pipe branches over the top and sides of the cake. Attach a few flowers and leaves to the branches. Press the Tootsie Roll circle on the front of the cake for a door hole and add the pretzel stick just below as the perch. Pipe yellow dots of frosting on the inside of the flowers. Use spatula to carefully stand the cake up. Place overlapping Chex squares in rows on the top of cake for a roof. Sprinkle the bran cereal around the base of cake.