Prepare the Chocolate Cake Batter: In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. In a separate bowl, combine the sugar, oil, buttermilk, eggs, and vanilla extract. Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Slowly pour in the boiling water, mixing until the batter is smooth.

Bake the Chocolate Cake: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch cake pans. Divide the batter evenly between the pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Cool the Cakes: Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

Make the Raspberry Puree: In a saucepan, combine the raspberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the raspberries have softened and released their juices, about 5-7 minutes.

Strain the Raspberry Puree: Press the raspberry mixture through a fine-mesh sieve to remove the seeds. Discard the seeds and set the smooth puree aside to cool.

Prepare the Gelatin: In a small bowl, sprinkle the gelatin powder over the cold water. Let it sit for 5 minutes to bloom (soften).

Make the Raspberry Mousse: Heat the bloomed gelatin in the microwave for 10-15 seconds, or until it's completely dissolved. Add the dissolved gelatin to the cooled raspberry puree and mix well.

Whip the Heavy Cream: In a large bowl, whip the heavy cream until stiff peaks form.

Combine the Mousse Ingredients: Gently fold the raspberry puree mixture into the whipped cream until just combined.

Make the Chocolate Ganache: Place the chocolate chips in a heatproof bowl. Heat the heavy cream in a saucepan until it just begins to simmer. Pour the hot cream over the chocolate chips and let it sit for 1 minute. Then, stir until the chocolate is completely melted and smooth.

Assemble the Cake: Place one cake layer on a serving plate. Spread a thick layer of raspberry mousse over the cake. Top with the second cake layer. Pour the chocolate ganache over the top of the cake, allowing it to drip down the sides.

Decorate and Chill: Decorate the cake with fresh raspberries and chocolate shavings, if desired. Refrigerate the cake for at least 2 hours to allow the mousse to set completely.