Preheat the oven to 350°F (175°C).

Prepare the 9-inch round baking pans by greasing and lining with parchment paper.

Whisk the sugar, flour, cocoa powder, chocolate pudding powder, baking soda, baking powder, and salt in a large mixing bowl.

Add the eggs, buttermilk, oil, and vanilla extract. Mix until smooth.

Carefully pour the boiling water into the mixture and slowly stir until fully combined—the batter will be thin.

Divide the batter evenly between the two prepared pans.

Bake for 25–30 minutes, or until the toothpick test comes out clean. You can also test by gently pressing the center of the cake with your finger. If it springs back, the cake is done.

Remove and cool completely in the pans on a wire rack. Once completely cooled, remove from the pans and set aside.

Whisk the eggs and sugar in a saucepan until combined.

Then turn on the heat to medium-low and gently heat while whisking constantly.

Slowly add 1 cup of the heavy cream until the mixture is warm and slightly thickened, about 3–4 minutes. Remove from heat.

Heat the other cup of heavy cream in a small saucepan until steaming. Remove from the heat and whisk in the chocolate chips, mixing until smooth and melted.

Slowly mix the smooth chocolate mixture into the egg mixture until combined.

Whip 1 cup of the heavy cream to soft peaks in a large chilled mixing bowl.

Slowly mix the chocolate mixture into the whipped cream, working in small batches until smooth.

Refrigerate the mousse for 15–20 minutes to firm up before assembling.

Place one cake layer in the bottom of a 10-inch springform pan.

Spread the chilled mousse on top in an even layer.

Carefully add the second cake layer on top. Press down gently.

Cover and refrigerate for 30 minutes while you make the ganache.

Place the chocolate chips in a heat-safe bowl.

Heat the heavy cream in a small saucepan over medium heat until just before it starts to simmer.

Pour over the chocolate chips 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.

Chill for another 30 minutes, or until set before serving.