In a saucepan, combine the chocolate milk, sugar, cocoa powder, and cornstarch. Stir gently over low heat until it thickens into a creamy consistency.

Remove from heat and add the unsalted butter, vanilla extract, and salt. Stir in the dark chocolate until melted and smooth.

Allow the custard to cool slightly before transferring it to the refrigerator, where it will become a silky center for your cake.

Preheat your oven to 175°C (350°F). Grease and line a cake pan with parchment paper.

In a mixing bowl, whisk together the room temperature milk, white vinegar, cane sugar, vegetable oil, and two teaspoons of vanilla extract until well combined.

Sift in the flour, cocoa powder, baking soda, baking powder, and salt, folding gently to maintain a light texture.

Slowly blend in the boiling water or freshly brewed coffee, creating a batter that is glossy and smooth.

Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Let cool completely before assembling.

Once the cake is cool, carefully slice it horizontally in half.

Spread a generous layer of the prepared chocolate custard on the bottom half, then carefully place the top layer over the custard.

Add more custard on top for a luscious finish, dusting with cocoa powder or a sprinkle of chocolate shavings if desired.