Preheat the oven to 180℃/355℉ . Line a 23cmx33cm baking tin with baking paper, over lapping 2 of the 4 sides (making for an easier release once baked). Give it a quick spray with Cook & Spray just to be sure. Set aside.
Start by blending the biscuits until fine or place in a Ziplock bag and crush using a rolling pin. Add to a medium bowl with the melted butter and mix until combined. Evenly spread onto the baking tray and place in the fridge until the sponge is ready.
In a large mixing bowl, add the softened butter and whip until lighter in colour. Add the sugar and whisk until even lighter in colour and mostly combined, about 3-4 minutes.
Add the egg whites and whisk until mostly combined, about 3 minutes. Add the flour, baking powder and salt. Do not yet whisk, first warm the milk mixture.
Add the milk to an microwave save pouring jug, add the vanilla essence and ground cinnamon. Whisk until mostly combined. Microwave for 90 seconds or until warm, stirring in between.
Give the dry and wet ingredients a rough whisk until roughly combined and add the warm milk mixture.
Remove the biscuit base from the fridge and pour the batter over the base. Evenly spread, ensuring the surface is even. Sprinkle over a generous amount of ground cinnamon or as desired. Note: the more cinnamon you add the more milktarty it'll be
Bake for 40-45 minutes or when poked with a skewer it comes out clean.
Allow to cool slightly before removing the sponge from the tray. Once the sponge has mostly cooled, you can start sectioning the sponge into rusks 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) ,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.
Once hardened, it's time to make that hot cup of coffee and get dunking! Geniet dit, julle :)