In a medium saucepan, whisk together milk, sugar, cocoa powder, and cornstarch over medium heat. Stir constantly for about 5-7 minutes until the mixture thickens and starts to bubble gently. Once thickened, remove from heat and stir in unsalted butter, vanilla extract, salt, and dark chocolate until smooth and glossy. Transfer to a bowl, cover, and chill in the refrigerator for 2-3 hours until it sets properly.

Preheat your oven to 350°F (175°C) and grease two 8-inch round cake pans, lining the bottoms with parchment paper. In a bowl, combine room temperature milk and white vinegar; let it sit for 5 minutes to curdle. Add vegetable oil, vanilla extract, and sugar; whisk until the mixture is well blended and smooth.

Sift together the all-purpose flour, cocoa powder, baking powder, and salt in a large bowl. Gradually add the dry mix to the wet ingredients, whisking until just combined. Pour in boiling water or freshly brewed coffee, mixing until the batter is silky and smooth.

Divide the cake batter evenly between the prepared cake pans, smoothing the tops. Bake in the preheated oven for 25-30 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow cakes to cool in the pans for about 10 minutes before transferring to wire racks to cool completely.

Once the cake layers are fully cooled, level them if needed with a serrated knife. Place one layer on a cake stand and spread a generous layer of the chilled chocolate custard on top. Gently place the second layer over the custard, applying a thin crumb coat with any remaining custard to seal in crumbs, then chill in the refrigerator for 30 minutes to firm up.

After chilling, apply a final, thicker layer of chocolate custard to the top and sides of the cake, using a spatula to smooth out the edges beautifully. For an added decorative touch, sprinkle crumbled cake trimmings around the sides. Refrigerate the cake for at least 30 minutes before slicing and serving.