Preheat your oven to 180°C (350°F) and prepare two 20cm (8-inch) cake pans.

In a large mixing bowl, beat butter, caster sugar, and vanilla extract until light and fluffy.

Add eggs one at a time, mixing thoroughly after each addition.

Combine cocoa and hot water in another bowl to create a smooth paste, then fold into butter mixture.

Add soured milk to the butter mixture.

Sift flour, baking powder, bicarbonate soda, and salt in a separate bowl, then combine with the batter until just mixed.

Pour the batter into prepared pans and bake for 23-25 minutes or until a toothpick comes out clean.

Allow cakes to cool for 10-15 minutes, then transfer to a wire rack to cool completely.

Melt sugars in a saucepan, then add butter and cream, stirring until smooth. Stir in salt and let cool.

Melt dark chocolate in a heatproof bowl, then whisk in heavy cream until smooth and glossy.

Beat softened butter, then gradually sift in icing sugar and mix with salted caramel until smooth.

Layer the cooled cakes with salted caramel buttercream and drizzle with caramel before pouring ganache on top.