Preheat oven to 170°C/325°F. Set a 23cm x 33cm baking tray aside.
Add sugar and eggs to a large bowl and using an electric whisk, beat until you leave a trail on the surface and it's light and pale in colour, when the whisk is lifted, about 5 mins.
Add the softened butter and whisk again until well combined about 1-2 mins.
In a small bowl, mix cocoa powder and very warm milk together. Once combined, add the mixture to the wet ingredients. Whisk until mostly combined. Note: the mixture will look like it's splitting but this is normal.
Add the flour, baking powder and salt. Using a strong spatula fold the dry ingredients into the mixture. Add the chopped chocolate and fold until mostly combined. Note: the mixture becomes quite pliable, so feel free to use your hands to mix the dough - yet you should resist overmixing.
Press the dough into your baking tray, ensuring the surface is even. Bake for 35-40 mins or until a cake skewer inserted in the middle comes out clean.
Remove from the oven and allow to cool down before sectioning. Once the sponge has cooled, use a serrated knife to section as follows, slice the sponge in have lengthwise, repeat width wise and continue halving each section until you're left with 32 sections (8 sections with x 4 rusks lengthwise).
Spread the sectioned sponge onto a baking tray that's lined with a wire rack. Dry in the oven, air fryer or dehydrator at 80-100℃/176-212℉ for 6 hours or overnight. Place a wooden spoon or cloth between the oven and its door, too allow unwanted moisture to escape. Note: the higher the temperature the faster they will dry of course. Also, the rusks continue to harden as they cool down.
Once they've cooled, it's time to put on the kettle and get dunking - the time to indulge is finally here! Te lekker.