Preheat oven to 350 F. Grease a rimmed cookie sheet (I use a 15x11-inch). Set aside.
Melt butter in a large saucepan.
Once butter has melted, stir in water and cocoa powder; bring to a boil.
Once at a boil, add buttermilk, and stir to combine.
Remove pan from heat.
Add the yogurt, eggs, and vanilla, and stir until well-combined.
Stir in sugar, flour, baking soda, and salt until just combined.
Bake for 15-20 minutes.
While the cake is baking prepare the icing.
For the icing, combine the butter, cocoa, and buttermilk. Bring to a boil.
Remove from heat, and add the powdered sugar, vanilla, and half of marshmallows. Stir to combine.
Sprinkle remaining half marshmallows over hot cake.
Pour icing over hot cake, and spread evenly over cake.
Sprinkle with chopped almonds.
Notes
Butter: I use salted butter in this recipe. If using unsalted butter, increase the salt in the cake to 3/4 teaspoon and add 1/4 teaspoon salt to the frosting.
Cocoa powder: I use unsweetened cocoa powder, not Dutch-processed cocoa powder, in this recipe.
Greek yogurt: I use fat-free. You could also use 2% or full-fat yogurt or full-fat sour cream.
Powdered sugar: If your powdered sugar is lumpy, sift it before you add it to the frosting. This will help the powdered sugar to better incorporate into the frosting so that you don't end up with lumpy frosting.