Step 1: Prepare the Chocolate Sponge

1. Preheat the oven to 170°C (340°F) fan and grease a 20 x 3.5 x 31.5cm cake tin. Line it with baking parchment.
2. In a jug, mix the milk, vinegar, vanilla extract, and oil. Let it stand for 5 minutes.
3. Sift together the flour, baking powder, cocoa powder, cornflour, sugar, and salt in a large bowl.
4. Gradually whisk the wet ingredients into the dry until smooth. Avoid overmixing.
5. Pour the batter into the prepared tin and bake for 18–20 minutes, or until springy to the touch.
6. Cool for 10 minutes in the tin, then transfer to a wire rack to cool completely. Slice into 12 rectangles.

Step 2: Make the Espresso Mixture

1. Dissolve espresso powder in hot water.
2. If using, mix in liqueur (e.g., rum, whiskey, or Irish cream).

Step 3: Prepare the Layers

1. Crush the shortbread biscuits into large crumbs using a rolling pin.
2. Microwave the caramel for 30 seconds to loosen it, then mix in the fleur de sel.
3. In a bowl, whisk together mascarpone, double cream, and icing sugar until thickened (1–2 minutes). Be careful not to overbeat.

Step 4: Assemble the Tiramisu

1. Dip each sponge rectangle briefly into the espresso mixture, then layer them in a deep dish (~30cm wide x 5cm deep).
2. Spread half of the mascarpone cream over the sponge layer.
3. Sprinkle half of the crushed shortbread crumbs and drizzle with half of the salted caramel.
4. Repeat the layers: espresso-soaked sponge, mascarpone cream, shortbread, and caramel.

Step 5: Finish and Chill

1. Dust the top generously with cocoa powder using a small tea strainer.
2. Chill the tiramisu for at least 4 hours, or overnight for the best flavor.