Let’s build this decadent cake layer by layer:

1. Make the Cake Layers:

• Preheat oven to 350°F (175°C). Butter and flour three 6-by-2-inch cake pans, lining the bottoms with parchment.
• In a small bowl, mix together the buttermilk, sour cream, coffee, and vanilla paste.
• In a large bowl, whisk together the cake flour, cocoa powder, baking powder, baking soda, and salt.
• In the bowl of a standing mixer with the paddle attachment, beat the butter and both sugars until light and fluffy.
• With the mixer on low, beat in the eggs and egg yolks, one at a time.
• Beat in the flour mixture and the buttermilk mixture in three alternating batches, beginning and ending with the flour mixture. Mix just until almost combined after each addition.
• Use a spatula to scrape the bottom and sides and gently complete the mixing. Do not overmix.
• Divide the batter evenly between the prepared pans; smooth the tops.
• Bake for about 30 minutes, or until a wooden skewer inserted into the center comes out clean.
• Let the cakes cool in pans for 10 minutes, then invert onto a wire rack, remove parchment, and let cool completely.

2. Make the Buttercream:

• In a large mixer bowl (paddle attachment), beat the room temperature butter until light and fluffy.
• Mix in the sifted powdered sugar, cocoa powder, vanilla, and salt on low speed until combined, then increase speed and beat until smooth.
• Add the cooled melted semi-sweet chocolate and cooled melted hazelnut chocolate. Beat until fully combined and fluffy.
• If the buttercream seems too thick, add heavy cream, one tablespoon at a time, until desired consistency is reached.

3. Make the Rocky Road Ganache:

• Place the semi-sweet chocolate chips in a medium heatproof bowl.
• Scald the heavy cream in a small saucepan over medium heat (bring it just to a boil).
• Pour the hot cream over the chocolate chips. Let sit undisturbed for two minutes.
• Whisk gently until completely smooth and glossy.
• Reserve about 2/3 cup of the plain ganache for the drip later (cover and keep at room temperature).
• Let the remaining larger portion of ganache cool slightly until it’s thickened but still stirrable. Fold in the toasted walnuts and mini marshmallows.

4. Assemble the Cake:

• Level the cooled cake layers if necessary.
• Place the first cake layer on your serving plate or cake board. Pipe a ring (dam) of buttercream around the top edge.
• Use an offset spatula to fill the center (inside the dam) with half of the rocky road ganache filling.
• Place the second cake layer on top. Pipe another buttercream dam. Fill with the remaining rocky road ganache filling.
• Place the third cake layer on top.
• Apply a thin crumb coat of buttercream over the entire cake and chill for 30 minutes.
• Frost the outside of the cake smoothly with the remaining buttercream.

5. Decorate:

• Gently reheat the reserved plain ganache in the microwave (50% power, short bursts) until it’s pourable but not hot.
• Drizzle the ganache over the top edge of the chilled cake, encouraging drips down the sides.
• Pile some extra rocky road ganache mixture (if you reserved any, or make a small extra bit) or simply some extra toasted walnuts and mini marshmallows decoratively on top of the cake.
• Add a final light drizzle of chocolate ganache over the topping, if desired.
• Chill briefly to set the drip before serving.

This multi-step assembly creates the final masterpiece!