Preheat your oven to 350°F (175°C). Grease and line the bottom of the cake pan with parchment paper.
In a mixing bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, combine buttermilk, vegetable oil, egg, and vanilla extract. Mix well.
Add the wet ingredients to the dry ingredients and mix until combined. Carefully stir in the boiling water until the batter is smooth.
Pour the batter into the prepared cake pan and bake for 25 minutes or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
For the mousse, melt the chopped chocolate using a double boiler or microwave until smooth. Allow to cool slightly.
In a mixing bowl, whip 1 ½ cups of heavy cream until soft peaks form. Refrigerate until ready to use.
In a separate bowl, beat the egg yolks with sugar and vanilla extract until pale. Gradually mix in the melted chocolate until well combined.
Fold the whipped cream into the chocolate mixture until fully incorporated.
In another bowl, whip the egg whites with a pinch of salt until stiff peaks form. Gently fold the egg whites into the chocolate mousse mixture.
Once the cake is fully cooled, slice it in half horizontally to create two layers. Place one layer on a serving platter.
Spread half of the mousse over the first layer of cake, then place the second layer on top and spread the remaining mousse over the top and sides of the cake.
Chill the cake in the refrigerator for at least 2 hours before serving to allow the mousse to set.