Step 1: Prepare the Chocolate Cake Base

1. Preheat your oven to 350°F (175°C). Grease and line the bottom of a 9-inch springform pan with parchment paper.

2. In a bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt. Set aside.

3. In a separate bowl, beat oil, granulated sugar, and brown sugar until well combined. Add eggs and vanilla, mixing until smooth.

4. Alternately add the dry ingredients and buttermilk to the wet mixture, starting and ending with dry ingredients. Mix until just combined.

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

6. Let the cake cool completely in the pan while preparing the mousse layers.

Step 2: Make the Raspberry Mousse

1. In a small saucepan, combine raspberries, sugar, and lemon juice over medium heat. Cook until the raspberries break down and release their juices (about 5 minutes).

2. Strain the mixture through a fine-mesh sieve to remove seeds.

3. In a small bowl, mix gelatin powder with 2 tablespoons of water. Let it bloom for 5 minutes.

4. Heat the strained raspberry mixture slightly, then stir in the gelatin until fully dissolved. Let it cool to room temperature.

5. In a chilled mixing bowl, whip heavy cream until soft peaks form. Gently fold in the raspberry mixture until smooth.

6. Spread the raspberry mousse evenly over the cooled chocolate cake base and refrigerate for 1 hour.

Step 3: Prepare the Chocolate Mousse

1. Melt chopped dark chocolate in a heatproof bowl over a double boiler or microwave in 20-second intervals, stirring until smooth.

2. In a small bowl, mix gelatin powder with 2 tablespoons of water and let it bloom for 5 minutes.

3. Heat ¼ cup of the heavy cream in a small saucepan, then stir in the bloomed gelatin until dissolved.

4. Whisk the gelatin-cream mixture into the melted chocolate until smooth. Let cool slightly.

5. In a separate bowl, whip remaining heavy cream with powdered sugar until soft peaks form.

6. Gently fold the whipped cream into the chocolate mixture until well combined.

7. Spread the chocolate mousse evenly over the raspberry layer and refrigerate for at least 2 hours.

Step 4: Make the Chocolate Glaze

1. Heat heavy cream in a small saucepan until steaming but not boiling.

2. Pour over chopped chocolate and let sit for 2 minutes. Add butter and stir until smooth.

3. Let the glaze cool slightly, then pour over the chilled cake. Smooth it out with a spatula.

4. Refrigerate for another hour until set.

Step 5: Garnish and Serve

1. Carefully remove the cake from the springform pan.

2. Decorate with fresh raspberries and chocolate shavings for an elegant finish.

3. Slice and serve chilled for the best texture and flavor.