Prepare the chocolate ganache and whipped salted caramel at least one day in advance so both frostings are cold and ready for assembly.

Preheat the oven to 180°C (360°F). Line a 9×13-inch rectangular cake pan with parchment paper.

Make the buttermilk by mixing the full-fat milk with vinegar or lemon juice. In another small bowl, mix hot water with instant coffee powder. Set both aside.

In a mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In a larger bowl, combine the egg, sunflower oil, vanilla extract, and prepared buttermilk. Mix until smooth.

Add the dry ingredients to the wet mixture and whisk only until just combined. Be careful not to overmix.

Pour in the hot coffee and mix until the batter is fully incorporated. The batter will be thin.

Transfer the batter to the prepared cake pan, spreading it evenly. Bake for 25-28 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 30 minutes, then gently unmold onto a parchment-lined tray. Allow it to cool completely before refrigerating for at least 1 hour.

Once chilled, use a sharp knife to cut the cake into three small rectangles.

Line a standard loaf pan with parchment paper, leaving an overhang for easy removal. Place the first cake rectangle in the pan, then spread a layer of chocolate ganache, followed by whipped salted caramel.

Repeat with the second layer, then top with the third cake rectangle. Press gently to compact the layers.

Chill for 2-4 hours to set the frosting.

Remove the cake from the loaf pan and place it on a serving plate. Cover the entire cake with a smooth layer of chocolate ganache.

Refrigerate overnight before slicing and serving for the best texture and flavor.