Make the Chocolate Sponge: Preheat the oven to 170ºC (340ºF) fan. Grease and line a 20 x 31.5cm cake tin with baking parchment. In a jug, combine milk, vinegar, vanilla, and oil. Let stand for 5 minutes. Sift the flour, baking powder, cocoa powder, cornflour, sugar, and salt into a large bowl. Whisk the wet ingredients into the dry ingredients until smooth (60-90 seconds). Pour into the prepared tin and bake for 18-20 minutes or until springy to the touch. Cool completely. Slice the sponge into 12 rectangles once cooled.

Prepare the Espresso: In a bowl, mix the espresso powder, hot water, and optional liqueur. Set aside.

Crush Shortbread Biscuits: Using the end of a rolling pin, crush the shortbread biscuits into pebbly crumbs (not too fine). Set aside.

Prepare Caramel: Warm the caramel in a microwave-safe bowl for 30-40 seconds. Stir in the sea salt.

Make Mascarpone Cream: In a large bowl, beat together the mascarpone, double cream, and icing sugar until thick but not overmixed (about 1-2 minutes).

Assemble the Tiramisu: In a large, deep dish (approx. 30cm x 5cm), soak each sponge rectangle in the espresso mixture for 5 seconds and arrange in a single layer. Spread half of the mascarpone cream over the soaked sponge layer, followed by half the shortbread crumbs and half the salted caramel. Repeat the layers, finishing with caramel on top.

Dust with Cocoa: Sift the cocoa powder over the top of the tiramisu. Cover the dish with cling film and refrigerate for at least 12 hours (up to 72 hours) to set and develop flavors.

Serve: Slice into portions and serve chilled. Enjoy!