Let’s assemble this multi-layered chocolate marvel:

1. Make the Chocolate Cake Layers:

Preheat oven to 350°F (175°C). Grease and flour/cocoa-dust three 6-inch round cake pans. Line bottoms with parchment.

In a large bowl, whisk together the flour, cocoa powder(s), baking powder, baking soda, and salt.

In a medium bowl, whisk together the buttermilk, sour cream, coffee (or hot water), and vanilla paste/extract.

In the bowl of a standing mixer fitted with the paddle attachment, beat the softened butter and both sugars until light and fluffy (3-5 mins).

With the mixer on low, beat in the eggs and egg yolks, one at a time, until incorporated.

Alternate adding the dry ingredients and the buttermilk mixture to the butter/egg mixture in three batches (begin and end with dry). Mix on low speed just until almost combined after each addition.

Use a spatula to scrape the bottom and sides and complete the mixing gently by hand. Do not overmix.

Divide the batter evenly between the prepared pans. Smooth the tops.

Bake for about 30-35 minutes, or until a wooden skewer inserted in the center comes out clean.

Let cakes cool in pans for 10 minutes, then invert onto wire racks, remove parchment, and cool completely. (Optional but recommended: Once cool, level tops with serrated knife).

2. Make the Chocolate Hazelnut Buttercream:

In the bowl of a stand mixer (paddle attachment), beat the room temperature butter until light and fluffy (3-5 mins).

Sift together the powdered sugar and cocoa powder. With the mixer on low, gradually add the sugar/cocoa mixture, alternating potentially with small splashes of heavy cream if needed for mixing.

Once incorporated, beat in the salt and vanilla extract.

Add the cooled melted semi-sweet chocolate and cooled melted hazelnut chocolate. Beat on low until combined, then increase speed to medium-high and beat until smooth and fluffy. Add more cream (1 Tbsp at a time) only if needed to reach desired consistency.

3. Make the Rocky Road Ganache Filling & Reserve Drip:

Place the semi-sweet chocolate chips in a medium heatproof bowl.

Scald the heavy cream in a small saucepan over medium heat (heat just until bubbles form around the edge, do not boil vigorously).

Pour the hot cream over the chocolate chips. Let sit undisturbed for 2 minutes.

Whisk gently until the ganache is completely smooth and shiny.

Reserve about ⅔ cup of the smooth ganache in a separate small bowl or squeeze bottle for the drip later. Let it cool and thicken slightly at room temperature.

To the remaining larger portion of ganache in the main bowl, let it cool until lukewarm (no longer hot, but still fluid). Gently fold in the toasted walnuts and mini marshmallows. Set aside briefly while assembling cake base.

4. Assemble the Cake:

Pipe a ring (dam) of the chocolate hazelnut buttercream around the top edge of the first cooled cake layer on your cake stand. This helps hold the chunky filling.

Use a spatula to fill the center inside the dam generously with the rocky road ganache mixture (walnuts & marshmallows).

Place the second cake layer on top. Repeat piping a buttercream dam and filling with the remaining rocky road ganache.

Place the third cake layer on top (upside down for a flat top).

Apply a thin crumb coat of the chocolate hazelnut buttercream over the entire cake. Chill for 20-30 minutes until firm.

5. Final Frosting & Drip:

Frost the entire chilled cake smoothly with the remaining chocolate hazelnut buttercream.

Chill again for 30 minutes until the frosting is firm to the touch.

Gently re-warm the reserved ⅔ cup ganache if it has become too thick (brief microwave bursts at 50% power or over warm water). It should be fluid but not hot.

Carefully apply the ganache drip around the top edge of the cold, frosted cake using a spoon or squeeze bottle. Fill in the top surface with ganache and smooth gently. Let set for a few minutes.

6. Final Decoration:

Pile some extra rocky road mixture (or just marshmallows/walnuts) artfully in the center on top of the ganache.

If desired, pipe decorative swirls or dollops of any remaining buttercream around the edge. Press in a few extra marshmallows and walnuts.

7. Chill and Serve:

Chill the finished cake for at least 30-60 minutes to fully set the ganache and frosting.

For best texture, let the cake sit at room temperature for 20-30 minutes before slicing with a large, sharp knife (dip in hot water and wipe clean between slices). Store leftovers covered in the refrigerator.