Preheat your oven to 180°C (350°F) and prepare your cake tins by greasing them with butter and lining the bottoms with parchment paper.

In a small bowl, whisk together the cocoa powder and hot water until smooth, allowing this mixture to cool slightly.

In a mixing bowl, beat the unsalted butter, granulated sugar, and light brown sugar together until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, ensuring each is well incorporated before adding the next.

In a separate bowl, combine buttermilk, vinegar, and cooled cocoa mixture. Gradually add this to the creamed butter mixture, alternating with flour and baking soda.

Divide the batter evenly between the prepared cake tins and bake for 30-35 minutes.

Let the cake layers cool in the tins for about 10 minutes, then transfer them to a wire rack to cool completely.

To make the fudge icing, melt the dark chocolate and then mix in the double cream, unsalted butter, icing sugar, vanilla extract, and salt until smooth.

Once the cake layers are cooled, frost with the creamy fudge icing.