Preheat your oven to 350°F (175°C) and line two 9-inch round baking pans with parchment paper. Lightly grease the sides.

In a large mixing bowl, whisk together the sugar, flour, cocoa powder, chocolate pudding powder, baking soda, baking powder, and salt. Add the eggs, buttermilk, oil, and vanilla extract. Mix until smooth. Carefully pour in the boiling water and mix until fully combined—the batter will be thin.

Divide the batter evenly between the two prepared pans and bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool completely in the pans, then remove and set aside.

To make the chocolate mousse, whisk the eggs and sugar together in a saucepan until combined before turning on the stove. Then turn on medium low heat and whisk constantly, adding 1 cup of heavy cream, until the mixture is warm and slightly thickened, about 3–4 minutes. Remove from heat.

In a separate saucepan, heat the other 1 cup of heavy cream until steaming. Remove from the heat and pour in the chocolate chips. Whisk together until smooth and completely melted.

Now take the set aside egg mixture and pour the chocolate into the pan, mixing until combined.

In a separate bowl, whip 1 cup of the heavy cream to soft peaks. Then whip the cream while adding in the chocolate mixture in batches until smooth. Refrigerate the mousse for 15–20 minutes to slightly firm up before assembling.

To assemble the cake, place one cooled cake layer in the bottom of a 10-inch springform pan. Spread the chilled mousse evenly over the top, then carefully place the second cake layer on top of the mousse. Lightly press to set. Cover and refrigerate for 30 minutes while you make the ganache.

For the ganache, heat the heavy cream in a small saucepan over medium heat until it just starts to simmer. Pour over the chocolate chips in a heatproof bowl and let sit for 1–2 minutes, then stir until smooth and glossy.

Pour the ganache over the top of the chilled cake, spreading it to the edges. Sprinkle a pinch of flaky sea salt over the top.

Refrigerate the entire cake for another 30 minutes, or until set. Slice and serve!