Preheat oven to 350 degrees. Butter and flour three 6-by-2-inch cake pans.
In a small bowl mix together the buttermilk, sour cream coffee and vanilla paste.
In a large bowl, whisk the flour, cocoa powder, baking powder, baking soda, and salt.
In a the bowl of a standing mixer beat butter and sugars until light and fluffy. With mixer on low, beat in eggs and yolks, one at a time. Beat in flour mixture and buttermilk in three alternating batches, beginning and ending with flour mixture.
Mix just until almost combined. Use a spatula to scrape the bottom and complete the mixing.
Divide batter between pans; smooth tops. Bake for about 30 minutes or until a wooden skewer inserted in the center comes out clean. Let the cakes cool in pans 10 minutes then invert onto a wire rack and let cool completely.
For the Buttercream
Beat the butter until light and fluffy. Mix in the confectioners' sugar, cocoa powder, vanilla, salt and melted chocolate. If buttercream is a bit thick you can add a tablespoon or two of heavy cream.
For the Ganache
Scald the cream in a small saucepan over medium. Bring it just to a boil then pour over the semisweet chocolate. Let sit for two minutes then whisk until smooth. Reserve about 2/3 a cup of the ganache for the drip. Mix in the walnuts and fold in the marshmallows once cool.
Pipe a ring of buttercream around the top edge of the first layer. Use a spatula to fill the center with the ganache. Place the next layer on top and repeat. Cover the outside with buttercream and smooth.
Drizzle the ganache on top of the cake. If it's thickened then you can mixx in a bit more cream and zap in the microwave for a few seconds at 50% power.
Pile the extra rocky road mixture on top of the cake and then press in a few extra marshmallows and walnuts and give a final light drizzle of chocolate.
If you’re not using 6-inch pans, double the recipe for 8-inch pans or triple the recipe for 9-inch pans.
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best, and easiest way to measure flour is by using a scale. If you don't have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
If you see little clumps of butter after you mix the wet ingredients don’t panic, it all works out by the time you mix in the dry ingredients. You can even use melted butter and warm the milk and sour cream up a bit if you like.
Sub warm water for the coffee if you're not a fan.
You can substitute whole milk yogurt for the sour cream if desired, I do it all the time and can’t really tell the difference.
To get FLAT layers that are moist inside and out try using cake strips! You can buy a set on theshop page or make your own from foil and paper towels at home. I made a wholeblog post on it so check it out if you’re interested!