Preheat the oven to 180℃/365℉ (no fan). Line a 23cmx33cmx6cm baking tin with baking spray or a very thin layer of butter.
In a large bowl, using an electric whisk, beat the eggs and gradually add the sugar. Continue whisking until pale in colour and fluffy, about 2-3 minutes.
Add the mascarpone and vanilla essence to the bowl and whisk until smooth, about 2 minutes.
Add flour, corn flour, baking powder and salt. Whisk until roughly combined and add milk. Continue mixing until fully combined but resist over mixing.
Pour 2/3rds of the batter into the lined baking tin and gently spread evenly, ensuring the edges aren't spread too thinly.
Mix coffee and 1 tbsp warm water together and allow to cool slightly. Add to the remaining batter and combine. Add dollops of the coffee infused batter on top of the vanilla batter.
Using a sift, dust a generous amount of cocoa powder on top. Using a butter knife or thin spatula, swirl the batters creating a marbled effect. The key is to create big swirls and not over mixing.
Give it a little shake to allow the batter to spread and pop in the oven. Bake for 40-45 minutes or until slightly cracked on top and golden brown.
Remove from the baked sponge from the oven and allow to cool slightly. Using a serrated knife, slice the rusks again, allowing for a smooth release.
Allow to cool slightly before slicing into rectangles and removing the sponge from the tray. Once the sponge has mostly cooled, you can start sectioning the sponge into rectangles using a serrated knife. Note: Aim for 4 rusks lengthwise and 8 rusks width wise (this is a personal preference but you can slice them thicker or thinner - it's up to you). The thicker they are the longer they will take to dry.
Drying: Preheat the oven to 90℃/194℉. Lay the sectioned sponge out onto a baking tray that's been lined with a rack, ensuring the tray isn't overcrowded. Place in the oven, with the door slightly ajar, using a cloth or wooden spoon, and dry for 6 hours or until fully dried (overnight can also be an option). Note: the rusks must be fully dried otherwise they'll slowly start to soften and stale.
