Combine C&H® Light Brown Sugar and oil. Beat with mixer until light and creamy.
Add eggs one at a time, mixing well after each egg. Scrape down bowl and mix until fully blended. While mixing on low, slowly add the water (it will be runny; don’t worry!)
Combine flour, salt, baking powder, baking soda and cocoa powder in a separate bowl. Alternate adding the dry ingredients with the buttermilk, starting, and ending with the dry ingredients. Don’t overmix! When you no longer see streaks of flour it’s done.
Grease your cake pans and dust with cocoa powder (You can give it one more spray of nonstick spray). Fill pans halfway full.
Bake at 325 degrees F (163 degrees C) for 35-40 minutes. You’ll know it’s done when the center springs back when gently touched and/or a toothpick inserted into the center comes out clean or almost clean (a few crumbs).
Cool in pans for 10 minutes then turn them out onto a baking rack until fully cooled.
Place room-temperature butter in your mixing bowl. Beat on medium-high until butter is lighter in color and creamy. With mixer on low, slowly add HALF of the powdered sugar. Mix until fully combined, scraping down the bowl if necessary.
Add the marshmallow crème, then with the mixer on low, add the rest of the powdered sugar, cream, vanilla, and salt. Whip 30-60 seconds until light and creamy.
Wait until the cake is completely cooled before frosting. It might even be a good idea to chill it for a few minutes. Using an offset spatula (or butter knife or spoon), gently spread the frosting over the top covering the cake completely. Then add garnishes.
Using frozen or very chilled cake layers, use a little buttercream to adhere a layer of cake to a cake board. Using an offset spatula, spread a generous layer of frosting covering the entire cake. Place another layer on top of frosting. Repeat, but turn the top layer of cake upside down before placing on top.
Lightly spread frosting on the entire cake, top and sides in a very thin layer. It’s okay if cake is showing through or there are crumbs. This is the crumb coat.
Place the cake in the freezer and freeze for a minimum of 10 minutes. After removing from freezer, spread a thick layer of frosting, covering the cake. Use a cake scraper (bench scraper) to go around the cake and scrape off the excess frosting. Patch any holes with more buttercream and scrape again. Repeat this process until you are satisfied with the smoothness of the cake.
Decorate as you wish! Immediately after frosting place a large candy cane on the side to look like a cocoa mug handle. Use smaller candy canes to decorate the sides. Continue to decorate with sprinkles, crushed candy canes, etc.
Place the cake in the freezer for a few minutes AFTER decorating the sides BEFORE decorating the top. Then top the cake off with additional decorations (frosting swirls, marshmallows, etc).