1. Preheat the oven to 350°. Set a 9-inch cake pan on top of a piece of parchment paper. Using the bottom of the cake pan as a guide, trace a circle on the paper with a pen or pencil. Cut out the circle and set aside. Repeat so you have two parchment circles. Use 1 tablespoon of the butter to grease two 9-inch cake pans, then place the parchment circles in the pans. Turn the parchment over so the greased side faces up.
2. In a standing mixer fitted with a paddle attachment, beat the remaining 1½ sticks of butter with the sugar on medium speed until light and airy, about 5 minutes. In a medium bowl, whisk together the flour, cocoa powder and salt and set aside. In another medium bowl, whisk together the Coca-Cola and milk.
3. Add the eggs, one at a time, to the butter-sugar mixture, using a rubber spatula to scrape down the sides of the bowl as needed. Turn the mixer to low and alternate adding the dry and wet ingredients in three batches, starting and ending with the flour mixture and mixing until just combined. Divide the batter between the two prepared pans. Place the cake pans in the oven and bake until a toothpick inserted in the center comes out clean, 25 to 30 minutes.
4. Remove the cakes from the oven and set aside to cool completely.
5. Make the frosting: In a standing mixer fitted with the paddle attachment, beat the butter at medium speed for 3 minutes. Meanwhile, in a medium bowl, sift together the confectioners’ sugar and cocoa powder. Turn off the mixer and add the sugar-cocoa mixture, then combine on low speed. Increase the speed to medium and add the vanilla, salt and milk. Mix for 3 minutes. Set aside until ready to use.
6. Remove the cooled cakes from the pans, peel off the parchment and place one layer on a serving tray or cake stand. Spoon and spread about one-quarter of the icing on top of the first layer, sprinkle with half of the marshmallows (if using), then top with the second cake layer. Use the rest of the icing to frost the top and sides of the cake (or just frost the top of the cake, as shown) and finish with the remaining marshmallows (if using), then garnish with chocolate curls or grated chocolate (if using).
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