Prepare the chocolate mousse first and place it in the refrigerator to chill. It can be made 1 to 2 days ahead.
Melt the chocolate chips in a heat-proof bowl using microwave increments of 20-30 seconds, stirring after each interval until smooth. Alternatively, use a double boiler.
Whisk the warm water and cocoa powder together in a small bowl until smooth. Pour this mixture into the melted chocolate and stir until well combined and slightly thick. Set aside.
In a large bowl, combine the cold heavy cream, powdered sugar, vanilla extract, and salt.
Whip the cream on medium-high speed until thick and holding its shape, about 2.5 to 3 minutes.
Gently fold the chocolate mixture into the whipped cream with a rubber spatula until just combined, being careful not to overmix.
Cover the mousse with plastic wrap and refrigerate for at least 2 hours.

Preheat your oven to 350 degrees F (175 degrees C). Grease and line four 8-inch cake pans.
In a large bowl, whisk together the flour, granulated sugar, sifted cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, whisk together the warm water, buttermilk, oil, and eggs until fully combined.
Add the wet ingredients to the dry ingredients and mix until just incorporated into the batter.
Divide the batter evenly among the prepared cake pans and bake for 23-26 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the cake layers cool in their pans for about 10 minutes before inverting them onto wire racks to cool completely.
Once cooled, level the tops of each cake layer with a serrated knife. If making ahead, wrap and freeze the cooled layers.

Beat the softened butter in a bowl with a paddle attachment on medium speed for 30 seconds until smooth.
Add the sifted cocoa powder, vanilla extract, and salt. Mix on low speed until everything is incorporated.
Gradually add the powdered sugar, mixing on low speed. Add the heavy cream halfway through mixing to help ease the process.
Stir in the cooled, melted chocolate on low speed until fully incorporated. Cover with plastic wrap to prevent a crust from forming and set aside.

Place the first cake layer on a greaseproof cake board, securing it with a small amount of buttercream.
Spread about 1/3 cup of raspberry jam on the first three cake layers, followed by about 1/3 of the chocolate mousse. If desired, gently press about 2/3 cup of fresh raspberries into the mousse. Place the top cake layer upside down for easier frosting.
Repeat this layering process with the remaining cake layers. Use a large offset spatula to smooth any overflowing mousse around the sides. Freeze the cake for about 10 minutes to firm up the filling, making it easier to frost.
Frost the entire cake with a thick, even layer of chocolate buttercream. Create decorative swoops on the frosting using an offset spatula or the back of a spoon. Top with a semicircle of fresh raspberries.