• Preheat oven to 350°F (175°C). Line the bottom of each cake pan with parchment paper and grease the sides.
• In a large bowl, mix sugar, flour, cocoa powder, baking powder, baking soda, and salt.
• Add eggs, buttermilk, oil, and vanilla extract to the dry ingredients. Beat until well combined.
• Gradually add boiling water or coffee, stirring until batter is smooth.
• Pour batter evenly into prepared pans and bake for 30-35 minutes or until toothpick comes out clean. Let cakes cool.
• For the mousse filling, sprinkle gelatin over cold water and let stand for 5 minutes. Microwave for 15 seconds and set aside.
• Whip cream and powdered sugar until stiff peaks form. Melt chocolate chips and combine with gelatin mixture. Fold into whipped cream until smooth.

Make the White Chocolate Mousse:

• Bloom gelatin in cold water for 5 minutes. Heat gently until dissolved.
• Melt white chocolate in a microwave or double boiler. Let cool slightly.
• Whip 1-1/2 cups of heavy cream with powdered sugar until soft peaks form. Fold in melted white chocolate and gelatin mixture. Chill for 15–20 minutes.

Assemble the Cake:

• To assemble, place one cake layer on serving plate. Spread mousse on top, then add second layer. Coat top and sides of cake with remaining mousse.
• Chill cake for at least 1 hour to set. Garnish with optional toppings before serving.