Preheat oven to 350F, grease three 6″ cake rounds. Dust with cocoa powder and line bottoms with parchment.
Place flour, sugar, cocoa powder, baking soda, baking powder and salt into the bowl of a stand mixer. Mix until well combined.
In a medium bowl, whisk eggs, oil, buttermilk, and vanilla. Very slowly pour in hot water while whisking constantly.
Add wet ingredients to dry and mix on medium for 2-3 minutes. Batter will be very thin.
Pour batter evenly into prepared pans. Bake for approx. 30-35 minutes or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10 minutes then turn out onto wire rack to cool completely.
Make the Honeycomb:
Line a quarter sheet pan (9” x 13”) with a silicone baking mat or tin foil (grease tin foil with cooking spray).*
Place the sugar, corn syrup, and water into a large saucepan. Stir well to combine and until the sugar is dissolved. Do not stir from this point forward. Wash down the sides of the pot with a pastry brush dipped in water, as needed, to prevent crystals from forming.
Bring to a boil over high heat and cook till the temperature on a candy thermometer reads 300F (this will take about 10 minutes).
Remove from heat, sprinkle sifted baking soda over top, and quickly whisk it in. The mixture will foam up (it will grow about 4 times in size) and turn golden. Don’t over wisk or it will deflate. Just whisk enough to incorporate baking soda (max 5 seconds).
Immediately pour into prepared pan and allow to cool (1-2 hours).
Once cooled, use the tip of a knife to break into shards of desired size.
Chop up some of the honeycomb into small pieces to add between the layers.
Store any extra pieces in an airtight container, separated by parchment, at room temperature.
Make the Chocolate Frosting:
In a medium bowl, whisk sifted sugar and cocoa powders to combine.
In the bowl of a stand mixer, beat butter until smooth (1 minute). Add in sugar mixture 1 cup at a time, mixing well after each addition.
Beat in melted chocolate. Add enough heavy cream until desired consistency is reached (I used 3 tablespoons).
Continue beating on high until smooth (approximately 5 minutes).**
Assemble the Cake:
Place one layer of cake onto a cake stand or serving plate.
Top with approximately 2/3 cup of buttercream and spread evenly. Sprinkle some crushed up honeycomb on top of the frosting***
Repeat with remaining layers and crumb coat the cake. Refrigerate for 30 minutes.
Frost the cake sides and top with a rustic pattern.
Place honeycomb shards along the top as desired. You can find tips for arranging them in this tutorial.
For thinner honeycomb use a half sheet pan. I made the recipe twice and used both sized pans so I could have a variety of thicknesses of honeycomb. You will have a lot left over if you do this! Store any extra pieces in an airtight container, separated by parchment, at room temperature.
The frosting will darken as it sits.
Ensure the honeycomb pieces are fairly small or it will be harder to cut the cake, especially if chilled.