To make the raspberry filling: Combine sugar, cornstarch, and water in a saucepan over medium heat, stirring to combine. Heat until wet and sugar begins to melt.
Add raspberries and stir to coat with the sugar mixture. Cook until raspberries soften and juice thickens, about 5 minutes. Strain to remove seeds if desired and refrigerate until cool.
To make the cake layers: Preheat oven to 350°F (176°C) and prepare three 8-inch cake pans with non-stick spray and parchment paper.
Combine flour, sugar, cocoa, baking soda, baking powder, and salt in a large mixer bowl.
Mix milk, vegetable oil, vanilla extract, and eggs in a medium bowl.
Add wet ingredients to dry ingredients and mix well. Slowly add hot water and mix on low speed until combined.
Divide batter among cake pans and bake for 22-25 minutes until a toothpick comes out with a few moist crumbs. Cool for 2-3 minutes before transferring to a cooling rack.
To make the chocolate frosting: Beat butter until smooth. Gradually add melted chocolate and mix until combined.
Add cocoa powder and mix well. Incorporate half of the powdered sugar, salt, and half of the heavy cream, mixing until combined.
Add remaining powdered sugar and mix until smooth. Adjust consistency with the remaining heavy cream.
To make the ganache: Place chocolate chips in a bowl. Heat heavy cream until boiling, then pour over chocolate. Let sit for 2-3 minutes, then whisk until smooth.
To layer the cake: Level the tops of each cake layer. Place the first layer on a serving plate, pipe a dam of frosting around the edge, and spread ganache in the center.
Drop spoonfuls of raspberry filling over the ganache and spread evenly. Repeat with the second layer.
Add third layer on top, frost the cake, and drizzle ganache over the top and sides.
Pipe swirls of frosting and decorate with raspberries. Refrigerate until ready to serve.
