Prepare the pan and preheat the oven. Grease and line the base and sides of a 23cm (9-inch) springform pan. Preheat the oven to 180°C (350°F).

Whip the eggs and sugar. In a medium-sized bowl, combine the eggs, light brown sugar, granulated sugar, and vanilla extract. Using a hand-held electric whisk, beat on medium-high speed until the mixture becomes light and creamy, about 4 minutes. When you lift the whisk, the mixture should leave trails for a few seconds.

Add the dry ingredients and fold through. Sift the plain flour, cocoa powder, baking powder, espresso powder, and salt into the egg mixture. Mix on low speed until just a few flour streaks remain.

Add the melted chocolate. Pour the melted dark chocolate into the batter and fold through with a rubber spatula, making sure to scrape the bottom of the bowl until smooth and fully combined.

Pour the batter into the pan and bake. Pour the batter into the prepared springform pan. Tap the pan firmly on the counter a couple of times to release air bubbles. Use an offset spatula to smooth out the surface. Bake for 15-20 minutes, or until the center springs back when lightly touched.

Cool the sponge. Remove the cake from the oven and place the pan on a wire rack to cool to room temperature. The top may look wrinkled. Gently press the top of the cake with a flat-based glass or measuring cup to create a flat surface. Cover and refrigerate until ready to use.

Bloom the gelatine. Place the lemon juice in a small bowl and sprinkle the powdered gelatine over the top. Stir with a fork and let it sit for a few minutes until the gelatine softens.

Blend the raspberries. Add the fresh raspberries to a blender or food processor and blend until smooth.

Pass through a sieve. Set a fine-mesh sieve over a saucepan and pour the raspberry puree into it, using the back of a spoon to push it through. Scrape the underside of the sieve and discard the seeds. You should have about 300 ml (1¼ cups) of raspberry pulp.

Heat and thicken the puree. Place the saucepan with the raspberry puree over medium-low heat. Add the granulated sugar and bring to a simmer, stirring frequently.

Add the gelatine. Stir in the bloomed gelatine mixture, heating gently (do not boil), until the gelatine dissolves. You may need to press the gelatine against the side of the pan.

Remove any lumps. Take the saucepan off the heat and pass the raspberry puree through a fine sieve set over a bowl to remove any remaining gelatine lumps (this step is optional if the gelatine has fully dissolved). Allow the puree to cool to room temperature, about 20 minutes.

Whip the cream and mix with the puree. In a separate bowl, whip the cold double cream using an electric hand mixer or stand mixer until soft peaks form. Gradually fold in the cooled raspberry puree in three parts, mixing until fully combined.

Set the mousse. Pour the raspberry mousse into the prepared springform pan over the cooled chocolate sponge. Smooth the top with an offset spatula. Cover and refrigerate for at least 3 hours, preferably overnight.

Remove the cake from the pan. When ready to serve, carefully remove the raspberry mousse cake from the fridge. Take off the springform pan sides and peel off the baking paper collar. If needed, smooth the edges of the mousse with a warmed offset spatula.

Decorate and serve. Transfer the cake onto a serving plate, decorate with fresh raspberries and chocolate shavings, then slice with a warm, dry, sharp knife. Serve immediately.