Adjust oven rack to middle position and heat oven to 325°F. Grease one 8-inch square baking pan and one 8-inch round cake pan, line each with parchment paper, grease parchment, and flour pans.
Sift flour, cocoa, baking soda, and salt together in a bowl. Microwave chocolate and butter in a microwave-safe bowl at 50% power, stirring occasionally, until melted (2 to 4 minutes). Whisk sugar, buttermilk, eggs, and vanilla together in a third large bowl.
Whisk chocolate mixture into sugar mixture until combined. Whisk in flour mixture until smooth. Divide batter evenly between prepared pans and bake until toothpick inserted in the center of each cake comes out clean, 35-45 minutes, switching and rotating pans halfway through baking. Let cakes cool in pans on wire rack for 10 minutes. Run a thin knife around the edge of the pans, remove cakes from pans, discarding parchment, and let cool completely on rack, about 1 hour.
Combine the egg whites and sugar in a large heat-proof bowl. Stir until smooth. Place the bowl on top of a pot of simmering water. Make sure that the bottom of the bowl does not touch the water. Stirring constantly, heat the egg white mixture to 160°F/71°C, 6-8 minutes (mini arm workout). Transfer the mixture to the bowl of a stand mixer fitted with a whisk attachment.
Whisk on medium until the mixture has enough air incorporated that you can turn the mixer up to high without splashing. Whisk on high until the mixture is silky, has still peaks, and is room temperature. This will take several minutes. Add in the vanilla and salt.
With the mixer running at medium-high, add in the butter one cube at a time. Wait to add the next until each cube is fully incorporated. Once all of the butter is added, mix until the texture is smooth and stable. Use a fine-mesh sieve to sift the berries into the butter cream (I actually skip this step and keep the seeds). Switch to a paddle attachment, and mix on low for a couple minutes more. *see note below for troubleshooting
Place one corner of the square cake against the lower edge of a large (about 16-inch diameter or wide) cake plate. Cut round cake in half. Place halves, with cut sides facing in, against top two edges of square cake to form a heart shape. If the cakes are not the same height, use a serrated knife to shave the tops of the cake until it is level. Spread 2 1/2 cups of buttercream over the top and sides of the cake in a thin, even layer. Fill a pastry bag fitted with a large star tip with the remaining buttercream. Pipe roses, spiraling from inside out over the top of the cake. (I had an Ateco 867, but the 827 would look more like roses than my swirls). Garnish with fresh raspberries between roses.
Meringue buttercreams can be tricky. Make absolutely sure to not get any egg yolk in with your egg whites. Even just the tiniest bit will cause you to have to start over.
If the mixture is too loose after adding the butter, cool it for a few minutes in the refrigerator and mix again. If the mixture is not smoothing out, let it sit at room temperature for a few minutes and then try again. It will eventually come together. Just keep watch over it so it doesn't then get over-mixed and separate.
If the Swiss Meringue is too daunting, I'm sure the American buttercream with freeze dried raspberries will also make a delicious cake. Just be sure to double that linked recipe.
Source: Cake: America's Test Kitchen; Buttercream: Amanda Faber