a lazy Susan (optional); 2 piping bags fitted with large round tips; a cake comb (optional); a silicone baking mat; thin wooden skewers
Put the cake on a slightly larger lazy Susan or cardboard cake plate.
Frost the top of the cake: Mix approximately three-quarters of the dulce de leche with 1/4 cup of the vanilla frosting; transfer to a piping bag fitted with a large round tip (this will be your darker frosting). Mix the remaining dulce de leche with the remaining 3/4 cup vanilla frosting; transfer to a piping bag fitted with a large round tip (this will be your lighter frosting).
Starting in the center of the cake, pipe concentric circles of the darker frosting out to about a 2-inch circle. Using a small offset spatula, spread out the frosting to about a 3-inch circle.
With the lighter frosting, pipe concentric circles around the 3-inch circle, leaving about a 1-inch margin around the edge of the cake. Using a clean offset spatula, smooth the lighter frosting to the edge of the cake, so the entire top of the cake is covered. Smooth everything with a clean offset spatula.
Line the point of a cake comb with the center of the cake and slowly rotate the cake to create concentric tree rings. When the lines of the circle meet, gently lift the comb (if you don't have a cake comb, this effect can be recreated using the tines of a fork).
Make the chocolate bark: Line a baking sheet with a silicone baking mat and set aside.
Separately melt the dark and milk chocolates over a double boiler or in the microwave. Pour the dark chocolate over the prepared baking sheet, using an offset spatula to spread to the width of the baking sheet but not smoothing out the chocolate. Drizzle the melted milk chocolate on top. Using the edge of a small offset spatula or a butter knife, make vertical lines and swirls running through the chocolate to create the look of tree bark. Be sure to apply various levels of pressure as the offset spatula is pulled through the chocolate to create a textured look. Let cool at room temperature until firmly set, at least 30 minutes.
Break the chocolate into large vertical pieces of varying heights to form pieces of bark. Use these pieces to cover the sides of the cake.
Make the berry mushrooms: Roll 1/2-inch pieces of marzipan between your fingers into stem shapes, making them wider at the base so they can stand upright. Place a stem inside the hollow of each raspberry and strawberry, using a skewer to secure the marzipan to the fruit if needed and to secure them down into the cake. Make mushrooms of various sizes and shapes to give a more natural effect and use the mushrooms to decorate the tops and sides of the cake, pressing gently to adhere the marzipan to the surface.
Make the pistachio moss and marshmallow stones: Put the pistachios in a mini food processor and add approximately 8 drops each of the green and yellow food coloring. Pulse the food processor to chop the pistachios to a coarse grind the texture of moss. Adjust the food coloring as needed to achieve the desired color. Spoon the moss around the cake plate alongside the bottom of the cake.
Mix the mini marshmallows and cocoa in a small bowl. Rough up the marshmallows between your fingers to create texture, shaking off the excess cocoa powder. Add the stones to the base of the cake on the pistachio moss.
Using a fine-mesh strainer, shake confectioners' sugar over the top of the cake, creating a snowy effect.