Preheat the oven to 175℃/347℉ and line a 23x33cm tray with cooking spray and set aside. Note: no need to line with baking paper, cooking spray should be enough to allow a smooth release once sectioned.
In a large bowl, whisk softened butter and 2/3 cup peanut butter together. Once combined add sugar, eggs, vanilla paste. Whisk until fully combined.
Add the flour, baking soda and salt. Whisk until roughly combined and add milk. Whisk until fully combined but do not overmix.
Spread the batter into the the prepared baking tin and even out the surface. Warm the remining 1/3 cup peanut butter in the microwave and add dollops on top of the batter. Use a knife or chopstick to roughly swirl the peanut butter batter together. Bake for 45 minutes. Note: the peanut butter that's layered on top will not fully harden during the drying process, so I prefer for there to not be too much peanut butter exposed on the surface of the batter but this depends on your personal preference.
Allow to cool down to the point where you can handle the tray with your bare hands. Using a serrated knife, mark 8 rusks on the width and 4 rusks on the length, yielding 32 rusks.
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.
Once she's fully dried it's time to make that warm cup of coffee and get dunking!
