Raspberry Compote

1. Place the fresh or frozen raspberries in a medium saucepan and cook over medium heat for about 10 minutes, stirring occasionally, until you get a thick raspberry compote.
2. Reserve about 80 g (1/3 cup) of the compote for filling the layer cake.
3. Blend the remaining compote with an immersion blender or a regular blender, then strain it through a fine sieve to get a smooth, seedless purée. Let it cool completely.

Raspberry Cake Layers

1. Preheat your oven to 165°C (325°F). Grease a heart cake pan with butter and line the bottom with parchment paper.
2. In a large mixing bowl or the bowl of a stand mixer fitted with the whisk attachment, beat the softened butter and sugar for 3 minutes until creamy and smooth.
3. Add the egg whites and vanilla extract, and beat for at least 5 minutes until the mixture is fluffy and volumized.
4. Incorporate the cooled raspberry purée and mix until the batter is uniform.
5. Gradually and alternately fold in the sifted dry ingredients— cake flour, baking powder, baking soda, and salt—and the buttermilk.
6. Gently mix with a spatula until you get a smooth pink batter. (Add a touch of pink or red food coloring if you want a more vibrant cake.)
7. Weigh the cake batter and divide it into 3 portions (bake in 3 separate batches using the same pan). Pour one portion into the prepared heart cake pan and bake in the preheated oven for 20–25 minutes without opening the door.
8. Check doneness by inserting a toothpick into the center; it should come out clean.
9. Run a knife gently around the edges of the cake to help release it from the pan, then let the cake cool upside down on a wire rack. (Once cooled, use a knife to carefully trim the crust if you want.)

Raspberry Buttercream

1. Prepare the raspberry buttercream according to the recipe and let it to room temperature before assembling the cake. Transfer the buttercream into a large piping bag.

Raspberry Mascarpone Frosting

1. In a medium mixing bowl, combine all the ingredients: cold mascarpone, cold heavy cream, cold raspberry purée, powdered sugar, and vanilla extract.
2. Whip with an electric mixer on low speed for 1–2 minutes until the frosting is smooth and firm.
3. Transfer the frosting to a piping bag and keep it refrigerated.

Assembling the Layer Cake

1. Place one cake layer on a cake board, then set it on a cake turntable.
2. Pipe a border of raspberry buttercream around the edges of the heart-shaped sponge.
3. Fill the center with a layer of raspberry mascarpone frosting, followed by raspberry compote.
4. Add the second cake layer and repeat the process with the final layer.
5. Coat the entire cake with a thin layer of raspberry buttercream (crumb coat) and smooth it with an offset spatula to seal in the crumbs.
6. Apply a second, thicker layer of buttercream and smooth it with a metal icing scraper.
7. Transfer the remaining buttercream into a piping bag fitted with a medium star tip.
8. Pipe a shell border along the top edge of the cake. Use the same tip to create another border along the bottom edge, then switch to a smaller star tip to pipe a second border for a vintage effect. (Add more pink food coloring to create contrasting shades for the borders if desired.)
9. Refrigerate the heart cake until ready to serve or enjoy immediately.