Let’s build this incredible Rocky Road Cake, layer by delicious layer! First, prepare the oven and pans. Preheat your oven to 350°F (177°C). Butter and flour three 6-inch round cake pans that are at least 2 inches deep. Alternatively, grease well, line bottoms with parchment, and grease parchment. Set aside.

Prepare the wet ingredients for the cake. In a small bowl or liquid measuring cup, whisk together the room temperature buttermilk, room temperature sour cream, cooled coffee, and vanilla bean paste (or extract). Set aside.

In a large bowl, whisk together the cake flour, sifted cocoa powder, baking powder, baking soda, and salt. Ensure all dry ingredients are well combined and free of lumps. Set aside.

In the bowl of your stand mixer fitted with the paddle attachment (or a large bowl with a handheld mixer), beat the softened butter, granulated sugar, and packed light brown sugar on medium-high speed until the mixture is very light, fluffy, and increased in volume, about 3-5 minutes. Scrape down the sides and bottom of the bowl.

Reduce the mixer speed to low. Add the room temperature eggs and egg yolks one at a time, beating well after each addition until fully incorporated. Scrape the bowl again.

Now, alternate adding the dry ingredients and the buttermilk mixture to the creamed butter-egg mixture, beginning and ending with the dry ingredients. Add about one-third of the dry ingredients and mix on low speed until just barely combined. Add about half of the buttermilk mixture and mix on low until just combined. Repeat with another third of dry, the remaining half of the buttermilk mixture, and finish with the last third of dry ingredients. Mix only until the ingredients are almost combined – stop when there are still a few streaks of flour.

Remove the bowl from the mixer (if using). Use a large rubber or silicone spatula to gently fold the batter a few times, scraping from the bottom, until the last streaks disappear. Do not overmix! Overmixing develops gluten and makes the cake tough.

Divide the finished batter evenly among the three prepared 6-inch pans. Smooth the tops gently with your offset spatula or the back of a spoon. Bake for approximately 30 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in their pans on wire racks for 10 minutes. Then, carefully invert the cakes onto the racks, remove parchment if used, and let them cool completely before proceeding.

While cakes cool, prepare the Chocolate-Hazelnut Buttercream. Ensure your melted semi-sweet chocolate and melted chocolate hazelnut spread have both cooled completely to room temperature. In a large bowl (or stand mixer bowl with paddle), beat the room temperature butter on medium-high speed for 3-5 minutes until very pale and fluffy. Add the sifted powdered sugar, sifted cocoa powder, salt, and vanilla extract. Start mixing on low speed until combined, then increase speed to medium-high and beat for another 2-3 minutes. Add the cooled melted semi-sweet chocolate and cooled melted hazelnut spread. Beat on medium speed until fully incorporated and smooth. If the buttercream seems too thick, beat in heavy cream one tablespoon at a time until desired consistency is reached. Set aside.

Next, make the Rocky Road Ganache. Place the semi-sweet chocolate chips in a medium heatproof bowl. In a small saucepan, heat the 1 1/2 cups of heavy cream over medium heat just until it comes to a simmer or gentle boil around the edges (scalded). Immediately pour the hot cream over the chocolate chips. Let the mixture sit undisturbed for 2-3 minutes to allow the heat to melt the chips. Gently whisk the mixture, starting from the center, until the ganache is completely smooth and glossy. Reserve about 2/3 cup of this plain ganache in a separate small bowl and set it aside for the drip later. Let the main bowl of ganache cool at room temperature until it’s thickened slightly but still stirrable (lukewarm or cool, not warm or hot). Once cooled, gently fold in the toasted, chopped walnuts and the mini marshmallows. If the ganache is too warm, the marshmallows will melt. Set this rocky road mixture aside.

Time to assemble! If needed, level the tops of your completely cooled cake layers with a serrated knife. Place the first cake layer on your serving plate or cake stand. Fill a piping bag fitted with a round tip (or just snip the corner) with some of the chocolate-hazelnut buttercream. Pipe a sturdy ring or “dam” of buttercream around the top outer edge of the cake layer. This will hold in the chunky filling.

Spoon about half of the rocky road ganache mixture into the center of the buttercream ring. Spread it gently to fill the dam. Carefully place the second cake layer on top. Repeat the process: pipe a buttercream dam and fill with the remaining rocky road ganache mixture. Place the third cake layer on top.

Apply a thin crumb coat of the chocolate-hazelnut buttercream all over the cake and chill for 15-20 minutes. Then, frost the entire outside of the cake (top and sides) smoothly with the remaining chocolate-hazelnut buttercream using your offset spatula.

Retrieve the reserved plain ganache for the drip. If it has thickened too much, gently warm it in the microwave for 5-10 seconds at 50% power, or stir in a tiny drop of warm cream, until it reaches a pourable drip consistency (cool but fluid). Carefully drizzle the ganache around the top edge of the chilled cake, letting it drip down the sides. Pour any remaining drip ganache onto the center top if desired.

Allow the drip to set slightly (5-10 minutes). Gently pile some of the leftover rocky road ganache mixture (if you reserved any, or make a little extra topping portion) decoratively on top of the cake inside the drip border. Press in a few extra mini marshmallows and toasted walnut pieces for garnish. You can even add a final light drizzle of the drip ganache over the topping, if you like. Chill the cake briefly before slicing.