For the cake:

1. Preheat oven to 350°F. Prepare four 4″ round cake pans and one 8″ square cake pan by greasing the pans, lining the bottoms with parchment paper, greasing the paper and flouring the pans. If using a high-quality baking spray, apply just before filling with batter.
2. In a medium bowl, combine the flour, cocoa powder, baking soda and salt.
3. In a separate bowl, microwave the chocolate and butter at 50% power, stirring occasionally, until melted (2-4 minutes).
4. In a third, large bowl, whisk the sugar, buttermilk, eggs and vanilla until combined. Whisk in the chocolate mixture. Whisk in the flour mixture until smooth.
5. Divide the batter between the pans, putting 143 g of batter into each of the 4″ round pans and the remaining 730 g into the 8″ square pan. That will give you the best chance of the cakes coming out at the same height, needing minimal trimming. Bake the small cakes for 20-25 minutes and the large cake for 35 to 45 minutes, Let cakes cool in the pans for 10 minutes, then run a thin knife around the edge of the pans before removing them from the pans and discarding the parchment. Let the cakes cool completely on a rack (at least an hour).

For the frosting:

1. In the bowl of a food processor, blitz the raspberries until smooth (about 30 seconds). Strain the purée through a fine-mesh strainer placed above a bowl; discard the solids.
2. In a medium bowl, microwave the white chocolate at 50% power, stirring occasionally, until melted and smooth (1-2 minutes). Let cool slightly.
3. Place the egg whites and sugar in a heatproof bowl (I use stainless steel) and whisk until combined. Place the bowl over a pan of simmering water, ensuring the bowl doesn’t touch the water. Continue to whisk until the mixture reaches 160°F or until it’s not grainy between your fingers.
4. Transfer the egg mixture to the bowl of a stand mixer, and beat with a whisk attachment until you get glossy, room temperature peaks. Tip: hold a couple of bags of frozen peas or corn against the outside of the bowl to bring the temperature down.
5. With the mixer running on medium-low, add small chunks of softened butter, one at a time. Take your time with this –  let the butter incorporate fully before you add the next piece (5 – 10 minutes). Don’t panic if the mixture gets soupy. Just keep beating. You can’t overbeat it and it will come together in the end. 
6. Add the chocolate and mix until just combined.
7. Remove 1 c of the frosting to a small bowl. You will tint this portion green to form the leaves on the cakes.
8. Add about 3/4 of the raspberry purée to the remaining icing in the stand mixture. Reserve the remaining purée to tint some of the frosting a darker shade of pink for some of the roses.

Assemble the cakes:

1. Using a serrated knife, level the cakes if needed. Cut the square cake into 4 equal squares. Cut each of the round cakes in half to make two half-moons. On four separate cake boards or pedestals, place two half-moons, cut sides in, against two sides of each square to make four heart-shaped cakes. Don’t worry about frosting the pieces together. Spread the tops and sides of the cakes with the frosting. 
2. Divide the remaining frosting and add the reserved raspberry puree to one half (you may not need all the reserved purée). Tint the white frosting with green gel food colouring (I used Americolor Gel Spruce).
3. Fit a small pastry bag with a leaf tip and fill it with green frosting. Fit two more small pastry bags with couplers and use a combination of tips #104 (small rose tip) and #123 (small curved Korean piping tip) to make the roses for the cake. I piped the roses onto small squares of parchment attached to a flower nail with a dab of frosting, then froze them for a few minutes before transferring them to the cakes. Pipe the leaves between the roses. 
4. Share and enjoy!