Preheat your oven to 350°F (175°C). Grease your dome-shaped cake pans.

In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, combine buttermilk, vegetable oil, egg, and vanilla extract. Stir until smooth, then add to the dry ingredients, mixing until just combined.

Divide the batter equally among the cake pans and bake for about 15-20 minutes or until a toothpick inserted comes out clean.

While the dome cakes cool, heat 1 cup of heavy cream in a microwave-safe bowl until simmering, then pour it over the chopped chocolate. Stir until thoroughly melted and let cool to room temperature.

In a separate bowl, beat the egg whites until soft peaks form, then gradually add sugar, beating until glossy and stiff.

Fold the chocolate mixture gently into the egg whites, then fold in the remaining heavy cream until smooth.

Once the dome cakes are cool, slice the tops to create a flat surface. Spoon chocolate mousse into each dome and place the cut-off tops back on.

Chill in the fridge for at least 2 hours before serving.