Preheat the oven to 350°F (175°C). Grease two 8-inch (20-cm) round cake pans and line the bottoms with parchment paper. Lightly grease the parchment.

In a large bowl, whisk together the flour, cocoa powder, sugar, baking soda, baking powder, and salt. Break up any remaining cocoa lumps so the mixture looks evenly colored.

In a medium bowl, whisk the eggs, buttermilk, vegetable oil, and vanilla until smooth. Pour into the dry ingredients and mix just until mostly combined.

Gradually whisk in the hot coffee: The batter will become noticeably thin and pourable; this is expected. Divide it evenly between the prepared pans.

Bake for 27 to 32 minutes, or until the tops spring back lightly and a toothpick inserted near the center comes out with a few moist crumbs rather than wet batter. Cool in the pans for 15 minutes, then turn the layers onto wire racks, remove the parchment, and cool completely, about 1 hour.

While the cakes cool, combine the raspberries, sugar, and lemon juice in a small saucepan. Cook over medium heat for 5 to 7 minutes, stirring and crushing the berries, until juicy and bubbling.

Stir the cornstarch and cold water together until completely smooth. Pour it into the bubbling raspberry mixture while stirring. Cook for another 1 to 2 minutes, until thick enough to coat a spoon. Transfer to a bowl and cool completely. It should become thicker as it cools rather than remain runny.

Beat the softened butter for about 2 minutes until creamy. On low speed, gradually add the powdered sugar, cocoa powder, and salt. Add the vanilla and ⅓ cup (80 ml) heavy cream, then beat at medium speed until smooth and fluffy. If the frosting is too stiff to spread, add additional cream 1 tablespoon (15 ml) at a time, using no more than 2 tablespoons (30 ml).

Once completely cool, trim any pronounced domes from the cake layers with a serrated knife. Do not assemble warm layers, or the frosting and filling can soften and slide.

Place one layer on a serving plate: Spread or pipe a thick ring of chocolate frosting around the outer edge to form a dam. Spoon the cooled raspberry filling inside the ring and spread it evenly without pushing it over the frosting border.

Place the second cake layer on top: Spread the remaining frosting over the top and sides with an offset spatula. If the layers begin sliding, refrigerate the cake for 15 minutes before continuing.

Refrigerate the assembled cake for 1 hour to firm the frosting and filling. Before serving, top with the optional fresh raspberries and dark chocolate. For a softer texture, let the cake stand at room temperature for about 20 minutes before slicing.