1. Preheat oven to 180°C (350°F). Grease, flour, and line two 20cm (8-inch) cake pans with baking paper.
2. In a large mixing bowl, beat butter, sugar, and vanilla extract until light and fluffy. Add eggs one at a time, beating well after each addition.
3. In a separate bowl, mix cocoa and hot water to form a thick paste. Gradually add the cocoa mixture to the butter mixture.
4. Add vinegar to milk and set aside to sour.
5. Sift together flour, baking powder, and bicarbonate soda. Gradually add dry ingredients to the butter mixture in three batches, alternating with the soured milk. Begin and end with dry ingredients.
6. Divide the batter evenly into prepared pans (1/4 of the batter per pan). Bake for 23 minutes, then let cool for 10–15 minutes before transferring to a wire rack. Repeat with the remaining batter.

Salted Caramel

1. In a heavy-bottomed saucepan, melt caster sugar and brown sugar over medium heat, stirring occasionally, until smooth.
2. Add butter and whisk until fully melted. Remove from heat and slowly pour in heavy cream, whisking continuously.
3. Return to heat and boil for 1 minute. Remove from heat, stir in salt, and let cool before use.

Dark Chocolate Ganache

1. Melt dark chocolate in a microwave-safe bowl in 30-second intervals, stirring between intervals.
2. Add heavy cream and stir until smooth and glossy. Let cool slightly until pourable but not hot.

Salted Caramel Buttercream

1. Beat butter until pale and fluffy. Gradually sift in icing sugar and continue to beat until fully incorporated.
2. Add 1 1/2 cups of cooled salted caramel and mix well. Refrigerate for 10–15 minutes to firm up for piping.

Assembly

1. Place one layer of chocolate fudge cake on a serving plate or cake board. Pipe a border of buttercream “kisses” around the edge using a large piping tip.
2. Fill the interior with salted caramel sauce, allowing some to drip over the edges.
3. Repeat layering with salted caramel buttercream, alternating with dark chocolate ganache and salted caramel sauce.
4. Top the final layer with piped buttercream and pour ganache over the top, encouraging it to drip down the sides.
5. Serve and enjoy!