For the cake
Adjust oven rack to middle position and heat oven to 325ºF.
Grease 8-inch square baking pan a d 8-inch round cake pan, line each with parchment paper, grease parchment, and flour pans.
Sift flour, cocoa, baking soda, and salt together in bowl. Microwave chocolate and butter in second bowl at 50 percent power, stirring occasionally, until melted and smooth, 2 to 4 minutes. Whisk sugar, buttermilk, eggs, and vanilla together in 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 center of each cake comes out clean, 35 to 45 minutes, rotating pans half-way through baking. Let cakes cool in pans on wire rack for 10 minutes. Run thin knife around edge of pans, remove cakes from pans, discarding parchment, and let cool completely on rack, about 1 hour.
For the frosting
Process 1½ cups raspberries in food processor until smooth, about 30 seconds. Strain purée through fine-mesh strainer into bowl; discard solids and set aside purée. Microwave chocolate in bowl at 50 percent power, stirring occasionally, until melted, 1 to 2 minutes; let cool slightly. Combine sugar, egg whites, and salt in bowl of stand mixer. Set bowl over saucepan filled with 1 inch barely simmering water, making sure the water does not touch the bottom of the bowl. Cook, whisking constantly, until mixture reaches 160 degrees, 5 to 8 minutes. Remove bowl from heat and transfer to stand mixer fitted with whisk attachment. Whip warm egg mixture on medium-high speed until stiff peaks form, about 5 minutes. Reduce speed to medium-low, add butter 1 piece at a time, and whip until smooth and creamy, about 2 minutes. Add melted chocolate and mix until just combined. Slowly add raspberry purée and mix until incorporated.
Place 1 corner of square cake against lower edge of large (about 16-inch diameter) cake platter. Using serrated knife, shave domed top from round cake to make it level with
square cake; discard top. Cut round cake in half. Place halves, with cut sides facing in, against top 2 edges of square cake to form heart shape. Spread 2½ cups frosting over top and sides
of cake in thin, even layer. Fill pastry bag fitted with star tip with remaining frosting and pipe roses (spiraling from inside out) over top and sides of cake. Place fresh raspberries between roses. Serve.