Preheat oven: First, preheat your oven to 350 degrees F and prepare three round cake pans with grease and flour.
Mix dry ingredients: Then, combine flour, cocoa powder, baking soda, baking powder, granulated sugar, and salt in a large mixing bowl or the bowl of a stand mixer.
Add the egg mixture: Next, add eggs, milk, vanilla, and oil to the dry ingredients and mix well.
Add water: After, slowly pour in the hot water while mixing on low speed. Keep increasing speed to medium until the chocolate mixture is smooth.
Divide and bake: Now, divide the cake batter into each prepared pan and bake for 25 to 30 minutes or until it passes the toothpick test. Let the cakes cool completely.
Make the chocolate fudge: While you wait, heat chocolate chips, butter, corn syrup, and heavy cream in a saucepan over low heat, stirring until smooth. Let melted chocolate mixture cool for 15 minutes.
Make the chocolate mousse: As that cools, melt dark chocolate in a heat-proof bowl over simmering water, or use a double boiler. Let it cool while you whip the vanilla and cream until soft peaks form. Then, gently fold the melted chocolate mixture into the whipped cream with a wooden spoon.
Make the white chocolate mousse layer: For the white chocolate mixture, melt the white chocolate the same way and let it cool while you whip the heavy cream and vanilla until soft peaks form. Then, fold remaining whipped cream into the melted white chocolate gently.
Assemble the choc mousse cake: To assemble, put one layer of chocolate cake on a serving plate as a chocolate cake base and spread half of the chocolate fudge on top. Add a chocolate mousse layer, then another layer of chocolate cake middle layer before topping with remaining chocolate fudge. Spread a layer of white chocolate layer on next and finish with the last cake layer. Top with a layer of Chocolate fudge.
Let it chill: Now, chill the triple chocolate cake in the fridge for at least two hours.
Serve: Finally, serve cooled cake with chocolate curls, shavings, or other chocolate decorations on top, if desired.