Step 1

Preheat oven to 350 degrees and place rack in middle of oven.

Step 2

Mix Coke and buttermilk together in a measuring cup, set aside.

Step 3

In a large mixing bowl, beat butter and sugars together until creamy. Add eggs and vanilla. Add the dry ingredients all at once. Mix and add the coke mixture all at once. Fold in the marshmallows.

Step 4

Spoon mixture into two greased and lined 9 inch cake pans or one greased 9x13 cake pan.

Step 5

Bake 30-35 minutes for layer cake and about 35-40 minutes for 9xCake is done when a few moist crumbs are attached to a toothpick.

Step 6

Remove from oven and if using layer pans, run a butter knife around edges and invert onto a rack to cool, placing the parchment side down so it won't stick to the rack. Make sure to peel parchment paper off before frosting cakes.

Step 7

Cool completely.

Step 8

Frosting: Mix the butter in a bowl with powdered sugar and cocoa. Add salt, vanilla and coke. Beat until creamy. Add milk until desired spreading consistency.

Step 9

Place cake on a platter. Spread 1/3 of frosting on layer. Top with other layer. Spread frosting on top and sides. Place chopped nuts on top of cake.