Preheat the oven to 180℃/356℉ and line a 23cmx33cm baking tin with baking paper. Set aside.
In a large bowl, using an electric whisk, whisk the butter, sugar and apricot jam together until creamed, about 1-2 minutes.
Add the eggs, one at a time, and whisk until the mixture turns paler in colour, about 1 minute.
Add the vinegar, vanilla essence and caramel essence and whisk until combined.
Add the flour, baking powder, bicarbonate of soda and salt, and whisk until large crumbs form. Pour in the milk and cream and whisk until fully combined and you're left with a smooth batter.
Pour the batter into the prepared baking tin and spread evenly. Bake for 35 minutes or until golden brown. Remove from the oven and allow to reach room temperature.
Once cooled, use a serrated knife to section the sponge. You'll aim to slice 4 sections from the length side of the tray and 7 sections from the width side, yielding 28 rusks in total. Note: I wouldn't cut them thicker than 4-5cm, otherwise they'll take forever to dry out. Also take note of the fact that the sponge will shrink while drying so take that into consideration as well. Evenly spread onto a cooling tray that's in a large baking tray. Allowing for some space around each rusk so that they dry properly.
Drying: Preheat the oven to 100℃/212℉. Bake for 2-3 hours with the oven's door slightly ajar, using a cloth. Rotate the rusks halfway through. Note: the rusks must be fully dried otherwise they'll slowly start to soften and stale.
Video
Notes
Storage: Store in an airtight container for 6 weeks to 2 months. But I promise you they won't last that long!