Preheat the oven to 175℃/347℉.
In a medium bowl, add butter and microwave until the butter has fully melted. Add the sugar to the warm butter and mix until mostly combined. Set aside.
In a large bowl, add flour, milk powder (if using), baking powder and salt. Mix lightly.
Add the butter-sugar mixture, an egg, and buttermilk. Mix until well combined. Note: feel free tip the mixture onto a clean working surface and kneading until it's fully combined.
Evenly spread the dough in a 23cm x 33cm baking tray using your hands. Note: this batter is just enough for a 23cm x 33cm. You can use a smaller tray but wouldn't recommend going bigger as your rusks will be very thin.
Bake for 45 minutes or until golden brown on top. Remove from the oven and allow to slightly cooldown. Using a serrated knife, mark 8 rusks on the width and 4 rusks on the length, yielding 32 rusks in total. Note: you can naturally cut them bigger or small depending on your preference.
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.
