Preheat the oven:
Preheat your oven to 180°C (350°F). Grease a large rectangular baking dish or two loaf tins.

Mix the dry ingredients:
In a large mixing bowl, combine the self-raising flour, sugar, and salt.

Combine wet ingredients:
In another bowl, whisk together the buttermilk, condensed milk, eggs, vanilla essence, and melted butter until smooth.

Make the dough:
Pour the wet mixture into the dry ingredients. Mix until well combined and no dry flour remains. The dough will be sticky and thick.

Bake the rusks:
Spoon the dough into the prepared tin(s) and smooth the top. Bake for about 1 hour, or until golden brown and a skewer comes out clean. Allow to cool slightly.

Slice and dry:
Once cool enough to handle, slice the baked loaf into fingers or blocks. Place on a wire rack or lined baking tray.

Dry out the rusks:
Reduce the oven temperature to 90–100°C (195–210°F). Dry the rusks in the oven for 6–8 hours (or overnight), turning once or twice, until completely dry and crisp.

Cool and store:
Let the rusks cool completely before storing in an airtight container.