Step 1

Preheat oven to 325F. Grease and flour 2 (8- or 9-inch) cake pans.

Step 2

To prepare cake, melt butter in saucepan, add marshmallows and stir until melted. Add chocolate and stir over low heat until melted. Add Coca-Cola, then set aside and let cool 10 minutes.

Step 3

Sift flour, cocoa powder, baking powder, baking soda and salt. Place sugar, oil and vanilla in a large mixing bowl. Beat with a mixer on medium speed. Add eggs one at a time, beating well after each addition. Add cooled chocolate mixture and beat on low until combined. Add half the flour mixture, then the buttermilk, then the remaining flour. Scrape down sides of bowl after each addition.

Step 4

Scrape into prepared cake pans. Bake 40 to 45 minutes, until cake springs back when touched lightly. Cool on wire racks. When completely cool, slice each layer in half horizontally to make 4 layers.

Step 5

To prepare icing, cream butter in bowl using a mixer until smooth. Add Coca-Cola and vanilla. Mix on low speed until blended. Add cocoa powder and chocolate. Mix until smooth, scraping down sides of bowl. Beat in powdered sugar, 1/2 cup at a time on low speed. Keeping mixer on low speed, beat until smooth. Icing is best if made one day ahead.

Step 6

Place 1 layer on a plate. Ice top. Repeat with remaining three layers and ice sides.