Melt the chocolate chips in a microwave-safe bowl, heating in 20-30 second intervals and stirring in between until smooth. If using a double boiler, melt over low heat. prep

In a small bowl, whisk together the warm water and cocoa powder. Add the mixture to the melted chocolate and stir until thick and smooth. Set aside. mix

In a large mixing bowl or stand mixer, combine the heavy cream, powdered sugar, vanilla, and a pinch of salt. Whip on medium-high speed for about 2 ½ to 3 minutes until the cream holds stiff peaks. prep

Gently fold the chocolate mixture into the whipped cream, using a rubber spatula. Be careful not to over-mix, or the mousse may deflate. mix

Cover the mousse with plastic wrap and chill in the fridge for at least 2 hours, or up to 2 days. prep

Preheat the oven to 350°F (175°C). Grease and line four 8-inch cake pans with parchment paper. prep

Sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt into a large bowl. mix

In a separate bowl, whisk together the warm water, buttermilk, oil, eggs, and vanilla until fully combined. mix

Add the wet ingredients to the dry ingredients and mix just until incorporated, being careful not to over-mix. mix

Divide the batter evenly between the pans and bake for 23-26 minutes, or until a toothpick inserted comes out with a few moist crumbs. bake

Allow the cakes to cool in the pans for about 10 minutes before transferring to wire racks to cool completely. prep

Once cooled, level the tops of the cakes with a serrated knife. If preparing in advance, wrap the layers and freeze them. prep

Beat the softened butter on medium speed for 30 seconds until smooth. prep

Add the sifted cocoa powder, vanilla extract, and salt. Mix on low until fully incorporated. mix

Gradually add the powdered sugar, mixing halfway through. Add heavy cream to ease mixing, and continue until fully combined. mix

Place the first layer of cake on a cake board and frost with a dab of buttercream to secure it. prep

Spread about ¼ cup raspberry jam on top of the cake layer. Then, add ⅓ of the chocolate mousse evenly on top of the jam. If desired, press in fresh raspberries. assemble

Repeat with the remaining layers, smoothing any excess mousse around the sides of the cake. Freeze for 10 minutes to firm up the mousse and frosting. assemble

Cover the cake with a thick, even layer of buttercream, smoothing it with an offset spatula or spoon. Decorate with a semicircle of fresh raspberries. finish