Preheat your oven to 350°F (without fan) and prepare three 8-inch cake pans by lining them with parchment paper and greasing with butter. In a large bowl, whisk together the flour, baking powder, baking soda, granulated sugar, light brown sugar, and salt until fully combined and lump-free. In a separate bowl, whisk the eggs, vegetable oil, buttermilk, and sour cream until smooth. Gradually add the dry ingredients to the wet mixture in two parts, gently folding and whisking just until combined—avoid overmixing. In a small bowl, combine the cocoa powder and instant coffee, then pour the boiling water over it and whisk until smooth and glossy. Fold this cocoa mixture into the batter until fully incorporated, forming a rich chocolate cake batter. Evenly divide the batter between the three prepared pans and bake for about 22-24 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached. Let the cakes cool in the pans for 20 minutes before transferring them to a wire rack to cool completely. For best results, refrigerate the cakes covered with cling wrap for about an hour.

Place the granulated sugar into a wide, non-stick pan and cook over the lowest heat setting. Allow the sugar to melt slowly, which may take 10-15 minutes. Once melting begins, gently stir with a clean spoon to ensure even melting; don’t worry about any hard lumps as they will dissolve as cooking continues. When the sugar has fully melted and turns a deep golden amber color, remove the pan from heat. Immediately add the room temperature butter and stir vigorously until it combines, even if the mixture looks slightly separated. Then slowly pour in the warmed heavy cream while stirring vigorously to create a smooth caramel sauce. Stir in the salt and vanilla extract. Transfer the caramel to a bowl or jar and allow it to cool at room temperature for about an hour, during which it will thicken.

Melt the dark chocolate using a double boiler or microwave in short 15-20 second bursts and set aside. In a stand mixer fitted with a paddle attachment, combine the room temperature cream cheese, sifted powdered sugar, and cocoa powder. Beat on medium speed until creamy, about 1-2 minutes, scraping the bowl as needed. Add half a cup of the cooled salted caramel sauce and mix on low speed until incorporated, scraping the bowl again. Pour in the melted chocolate (still warm) and whisk on low until fully combined. Finally, add the cold heavy cream and mix until smooth, scraping the sides as necessary. The frosting should be smooth but slightly runny; avoid overmixing. If any chocolate lumps appear, remove a small amount of frosting, microwave briefly for 20 seconds, then mix it back into the batch. Chill the frosting for 15-20 minutes until it becomes spreadable.

Once the cakes and frosting are cooled, trim the domed tops off each cake layer carefully using a serrated knife. Place the first cake layer on a turntable, spread a layer of frosting on top, then pipe a border of frosting around the edge to create a “dam.” Pour 3-4 tablespoons of salted caramel sauce inside the dam and spread evenly—this prevents the caramel from spilling over. Repeat this process with the second layer, then place the third cake layer on top. Cover the entire cake with a thin crumb coat and chill for 20 minutes. After chilling, apply the remaining frosting evenly all over the cake. Drizzle any leftover caramel sauce over the top as desired. Serve and enjoy!