Preheat the oven to 350 degrees F. Grease three 8-inch round cake pans. Line with parchment paper, then butter/spray with cooking spray.

In a medium bowl combine the flour, sugar, unsweetened cocoa powder, baking soda, baking powder and salt.

In the bowl of a stand mixer (or use a hand held mixer) beat together the eggs, buttermilk, greek yogurt, canola oil and vanilla until smooth. Slowly add the dry ingredients to the wet ingredients with the mixer on low until there are no longer any clumps of flour. Add the cream soda (or ginger beer) and mix until combined. The batter should be pourable, but not super thin. Divide the batter evenly among the 3 cake pans. Transfer to the oven and bake 20 to 25 minutes, until the tops are just set and no longer wiggly in the center. Remove and let cool five minutes, then run a knife around the edges of the pan. Grab 3 large flat plates, line them with wax or parchment paper and invert the cakes onto the paper lined plates. Cover and let the cakes cool completely before frosting.

To make the frosting. Add the butterscotch chips and cream to a small saucepan. Set over low heat, stirring often until melted and smooth. Remove from the heat. Set the butterbeer sauce aside.

Add the butter and powdered sugar to the bowl of a stand mixer (or use a hand held mixer). Beat the butter, peanut butter and powdered sugar together until the butter is light and fluffy, about 4 minutes. Add the cocoa powder and vanilla and beat, scraping down the sides as needed another 2 minutes or until there are no streaks of white. Add 1/2 cup of the butterbeer sauce and whip the frosting for 2-4 minutes or until light and fluffy.

To assemble the cake. Place 1 cake layer, flat side up, on a plate or cake stand. With a knife or offset spatula, spread the top with frosting. Drizzle with 2-4 tablespoons butterbeer sauce. Place the second layer on top, rounded side up, and spread the frosting. Drizzle with 2-4 tablespoons butterbeer sauce. Add on the final layer, rounded side up, and top with the remaining frosting, evenly spreading on the top and sides of the cake. Drizzle the cake all over with the remaining butterbeer sauce.

To make the web: Melt the marshmallows in the microwave on 30 second intervals, stirring after each, until melted and smooth. Let cool 3-5 minutes and then use your hands to stretch/pull the melted marshmallow around the cake to create a spider web. Go in as many directions as possible. It will be messy, but fun! Cake should be kept in an airtight container in a cool, dry place or in the fridge.

To make the spiders. Melt together the chocolate chips and peanut butter in the microwave or over a double broiler. Stir in the crushed pretzel sticks. Mound the pretzels into tablespoon size balls and place on a parchment or wax lined baking sheet. Take the remaining pretzels and toss them through the melted chocolate. Attach 3 pretzel legs on either side of the pretzel balls and then add 2 candy eyes to create eyes for the spiders. Place in the freezer to harden for 5-10 minutes.

To create the chocolate trees, melt 8 ounces semi-sweet chocolate and let cool 5 minutes. Transfer the chocolate to a ziplock bag and snip a small portion of the corner off the bag. On a parchment lined baking sheet, pipe out skinny leafless trees. Place in the freezer to harden for 5-10 minutes.

Just before serving the cake, stick the trees around the top of the cake and decorate with chocolate spiders. Serve the cake at room temperature.