Preheat your oven to 350°F (175°C) and grease an 8-inch round cake pan with butter and flour to prevent sticking.

In a mixing bowl, combine all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, cream softened unsalted butter with granulated sugar until fluffy.

Beat in large eggs one at a time, then add vanilla extract.

Gradually mix in the dry ingredients alternately with buttermilk until just combined.

Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick comes out clean.

Cool for 10 minutes before transferring to a wire rack to cool completely.

While the cake is cooling, prepare the raspberry mousse by pureeing fresh raspberries in a blender and straining the mixture.

In another bowl, whip heavy cream and vanilla extract until soft peaks form, then gently fold in cooled melted dark chocolate and strained raspberry puree.

Chill the mousse in the refrigerator for about 30 minutes to set.

Once the cake has completely cooled, slice it horizontally to create two equal layers.

Spread half of the raspberry mousse over the bottom layer, then place the second layer on top and spread remaining mousse.

Chill the assembled cake in the refrigerator for at least one hour.

To make the ganache, heat heavy cream in a saucepan until it reaches a boil, then pour it over chopped dark chocolate and stir until smooth.

Drizzle the ganache over the chilled cake for an elegant finish.

Slice the cake and serve with a dollop of whipped cream or fresh raspberries.