Set your oven to 350°F (175°C) and grease a baking tray.
In a bowl, combine flour, sugar, salt, and baking powder. In another bowl, whisk buttermilk, melted butter, and egg until smooth.
Add the wet ingredients to the dry and mix gently until a soft, slightly sticky dough forms.
Press the dough evenly into the tray, smoothing the top with damp fingers.
Bake at 350°F (175°C) for 1 hour, or until golden brown and a toothpick comes out clean.
Let the slab cool for 10–15 minutes, then slice into fingers or squares. Lower the oven to 200°F (95°C). Spread the slices cut side up on a tray.
Bake for 4–6 hours, flipping halfway through, until the rusks are dry and golden.