Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Stir to combine.

In a separate bowl, whisk together buttermilk, vegetable oil, egg, and vanilla extract. Slowly add the wet ingredients to the dry ingredients, mixing until just combined.

Carefully stir in the boiling water until the batter is smooth. Pour the batter into the prepared cake pan.

Bake for 25 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

For the mousse, place the chopped chocolate in a double boiler or microwave-safe bowl. Melt the chocolate, stirring until smooth. Allow to cool slightly.

In another bowl, whisk the eggs, sugar, vanilla extract, and a pinch of salt until thick and light in color. Gently fold in the melted chocolate.

In a separate bowl, whip the heavy cream until soft peaks form. Carefully fold the whipped cream into the chocolate mixture until fully combined.

Slice the cooled cake in half horizontally to create two layers. Place one layer on a serving plate and spread half of the mousse on top. Place the second layer on top and spread the remaining mousse over the cake.

Chill the cake in the refrigerator for at least 2 hours before serving. Decorate with whipped cream and chocolate shavings if desired.