For the cake: Firstly, preheat the oven to 175C/Gas Mark 4 and grease and line three 8-inch cake tins. In a small bowl sift together the flour, baking powder, salt and espresso powder and set aside.
Then using an electric mixer, beat the butter for a few minutes until smooth. Add both the sugars and increase the speed to medium-high. Keep mixing until the butter and sugars are creamed together and the mixture is light and fluffy. With the mixer on low, add in the vanilla extract followed by the eggs one at a time making sure they are each fully incorporated.
Then in alternating batches, add in half of the dry ingredients, the whole milk and the remaining dry ingredients mixing on low in between. Only mix until the batter is just combined. Then with the mixer on low, stream in the hot coffee and mix until smooth.
Evenly distribute the batter between the three pans, and bake for 24-26 minutes until a cocktail stick inserted into the centre of the cakes comes out clean. Let the cakes cool in the tins for 10 minutes on a wire rack, before removing the cakes from the tins.
For the coffee soak: Pour all the ingredients into a saucepan and place over a medium-high heat. Bring the mixture to a slight boil, then lower the heat and simmer for about 5-10 minutes. Remove from the heat and set aside to cool until ready to use.
For the swiss meringue buttercream: Place the egg whites and sugar in the bowl of an electric mixer and gently whisk by hand to combine. Then place over a saucepan of simmering water, making sure the bottom of the bowl doesn't touch the water. Whisking occasionally heat the egg mixture until it reaches 160F (70C) on a candy thermometer. Once you've reached the right temperature, take the bowl off the heat and fit onto a stand mixer. Using the whisk attachment, whisk the mixture for 8-10 minutes until it holds medium-stiff peaks. When done, the outside of the bowl should have reached room temperature. Stop the mixer and swap the whisk attachment out for the paddle.
Then with the mixer on low, add in the butter a few cubes at a time, then the vanilla and finally the caramel. Once all incorporated, turn the mixer up to medium-high and beat for 3-5 minutes until the buttercream is silky smooth.
To assemble the cake: Place one of the sponges onto your serving plate, brush liberally with the coffee soak and spread on 1/4 of the buttercream. Repeat with the next sponge, and then place the final sponge on top brushing with the remaining coffee soak. Cover the entire cake with the remaining buttercream, smoothing or swirling as desired. Finally place the remaining caramel into a piping bag and drizzle over back and forth.