1. Prepare the Sponge Cake Base:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper to prevent sticking.
2. Whisk the Eggs and Sugar: In a large mixing bowl, whisk the eggs and granulated sugar together until light and fluffy, about 3-5 minutes. The mixture should pale in color and form ribbons when the whisk is lifted.
3. Add Vanilla Extract: Stir in the vanilla extract.
4. Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt.
5. Fold in the Dry Ingredients: Gently fold the dry ingredients into the egg mixture in batches, being careful not to deflate the batter.
6. Add Butter: Gently fold in the melted butter to the batter.
7. Bake the Cake: Pour the batter into the prepared cake pan and bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
8. Cool the Cake: Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

2. Make the Chocolate Mousse Layer:

1. Melt the Chocolate: Break the dark chocolate into pieces and melt it gently using a double boiler or in the microwave in 30-second intervals, stirring in between.
2. Bloom the Gelatin: In a small bowl, sprinkle the gelatin powder over the cold water. Let it sit for 5 minutes to bloom.
3. Heat the Gelatin: Heat the gelatin mixture in the microwave for 10-15 seconds or until fully dissolved.
4. Whip the Cream: In a chilled bowl, beat the heavy cream with the powdered sugar and vanilla extract until stiff peaks form.
5. Combine Chocolate and Gelatin: Stir the melted chocolate and gelatin into the whipped cream until fully incorporated.
6. Assemble the Chocolate Mousse Layer: Pour the chocolate mousse mixture onto the cooled cake base. Smooth it out evenly and refrigerate for at least 2 hours, or until set.

3. Make the Raspberry Mousse Layer:

1. Prepare the Raspberry Puree: If using fresh raspberries, blend them into a puree using a food processor or blender. Strain the puree to remove the seeds, if desired. In a small saucepan, heat the raspberry puree with sugar and lemon juice over low heat until the sugar dissolves.
2. Bloom the Gelatin: As with the chocolate mousse, bloom the gelatin in cold water, then heat it to dissolve.
3. Whip the Cream: In a separate bowl, whip the heavy cream and powdered sugar until soft peaks form.
4. Combine the Puree and Gelatin: Stir the dissolved gelatin into the raspberry puree mixture and allow it to cool to room temperature.
5. Fold in the Whipped Cream: Gently fold the whipped cream into the raspberry mixture until smooth and fully combined.
6. Assemble the Raspberry Mousse Layer: Spoon the raspberry mousse on top of the set chocolate mousse layer. Smooth the top and refrigerate for at least 2 more hours, or overnight for the best results.

4. Prepare the Raspberry Coulis (Optional):

1. Make the Coulis: In a small saucepan, combine fresh raspberries, sugar, and lemon juice. Cook over medium heat until the raspberries break down and form a sauce, about 5-7 minutes.
2. Strain the Coulis: Strain the mixture to remove the seeds and set it aside to cool.

5. Assemble and Serve:

1. Remove the Cake from the Pan: Once both mousse layers have set, carefully remove the cake from the springform pan.
2. Decorate: Garnish the top of the cake with fresh raspberries and dark chocolate shavings. You can also drizzle with raspberry coulis for an extra burst of flavor.
3. Serve: Slice and serve the Raspberry Chocolate Mousse Cake chilled for the best texture.