Combine the gelatin and ¼ cup of water in a small bowl and mix together. Set aside.

Combine the raspberries, sugar and ¼ cup of water in a medium saucepan. Bring the raspberries to a boil and simmer stirring occasionally for 10 minutes, until the berries break down. Transfer the raspberry mixture to a food process or blender and purée until the raspberries are smooth. Pour the puréed raspberries through a fine mesh strainer to remove the seeds, pushing with a wooden spoon or rubber spatula. Return the raspberries to the saucepan and add the soaked gelatin, stirring over low heat until well combined. Pour the raspberry mixture into a large bowl and let it cool to room temperature.

Whip the heavy cream with an electric mixer into stiff peaks. Fold the whipped cream into the raspberry mixture and chill for at least 4 hours, until the mousse sets up.

Make the cake layers. Start by pre-heating the oven to 350°F and greasing two 9-inch cake pans with butter. Line the pans with parchment paper and lightly grease the parchment paper too.

Break the chocolate into pieces. Melt the chocolate in the microwave by cooking and stirring in 30-second increments or in a double boiler over simmering water.

Combine the flour, sugar, baking powder and salt in a large bowl. Add the eggs and melted butter, and stir until combined. Pour the melted chocolate, milk and boiling water and vanilla extract into the cake batter and stir until smooth. Transfer the batter into the prepared cake pans and place the pans in the oven.

Bake at 350°F for 35 to 40 minutes, until a toothpick inserted into the center of the cake comes out clean. Transfer the pans to a cooling rack. Cool for 15 minutes, then unmold the cake layers onto the rack and cool completely.

Assemble the cake. Trim the top of cake layers so that both the tops and bottoms of each layer are flat. Place one layer, cut side down, on serving plate or cake stand. Whisk the chilled mousse to loosen it up and spread the mousse evenly over the cake layer. Dip an offset metal spatula in hot water and run the spatula over the top and edges of the mouse to smooth it out. Place the second cake layer, cut side down, on top of the mousse.

Make the ganache. Chop the dark chocolate into chunks and place it in a medium bowl. Bring the heavy cream to a simmer in a saucepan, but do not boil. Pour the warm heavy cream over the chocolate chips and stir into a smooth and shiny consistency. Slowly pour the ganache into the center of the cake. In a circular motion, spread the chocolate out so it covers the top of the cake and drips down the sides of the cake. Try to do this in one step to get a nice smooth top to the cake. With an off-set spatula or knife, smooth out the chocolate on the sides of the cake. If you want a totally flat edge, dip the spatula in hot water to smooth out the chocolate. Garnish with some fresh raspberries if desired.