Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, mix buttermilk, vegetable oil, eggs, and vanilla. Combine with dry ingredients until just mixed.
Gradually stir in boiling water until smooth. Pour batter into prepared pans and bake for 30-35 minutes.
Cool cakes for 10 minutes before transferring to wire racks.
For mousse: Cook raspberries with sugar and lemon juice until saucy; strain seeds. Bloom vegan gelatin in cold water; dissolve in warm raspberry puree. Fold into whipped cream.
Assemble by layering cake with mousse; frost with ganache made from heated cream poured over chopped chocolate.