Make the cake base: Preheat your oven to 350°F (175°C). Grease and line the bottom of a 9-inch springform pan with parchment paper. In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda, salt, and sugar. Add in oil, vanilla, eggs, and buttermilk, then whisk until the batter becomes smooth and lump-free. Pour into the prepared pan and bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean. Let it cool completely inside the pan.
Prepare the raspberry layer: In a small saucepan, combine raspberries, sugar, and lemon juice. Cook over medium heat for 5–7 minutes until the berries break down and the mixture thickens slightly. Strain through a fine mesh sieve to remove seeds if desired. In a separate small bowl, mix gelatin with cold water and let it bloom for 5 minutes. Stir the gelatin mixture into the warm raspberry purée until dissolved completely. Let it cool slightly before pouring over the cooled cake base. Refrigerate for 30–40 minutes until the layer is firm.
Make the dark chocolate mousse: Place the chopped dark chocolate and butter in a heatproof bowl. In a small saucepan, bring 1/2 cup of cream just to a simmer, then pour it over the chocolate. Let sit for a minute, then stir gently until melted and silky. In another large mixing bowl, whip the remaining 1 1/2 cups of cream with sugar and vanilla until soft peaks form. Gradually fold the whipped cream into the melted chocolate in three additions, being careful not to deflate the mousse. The texture should be rich and airy.
Assemble the cake: Pour the dark chocolate mousse evenly over the set raspberry layer. Use an offset spatula to smooth the top. Lightly tap the pan on the counter to release any trapped air bubbles. Cover with plastic wrap and chill in the refrigerator for at least 4 hours or overnight until firm.
Serve: Once fully chilled, carefully release the springform pan and transfer the cake to a serving plate. Decorate with fresh raspberries, chocolate shavings, or a dusting of cocoa powder. Slice with a warm knife for clean, neat cuts and serve immediately.
