Preheat the oven to 200°C/390F, or 180°C fan. Grease and line a 9"x9" (23cmx23cm) square tin.
Weigh out the flour, and add in the salt and baking soda. Whisk through to remove any lumps and distribute the ingredients through the flour.
500 grams self-raising flour, ½ teaspoon table salt, ½ teaspoon baking soda
Melt the butter, then add in the sugar and whisk through until completely dissolved. The sugar may need to be heated with the butter to completely dissolve.
65 grams white granulated sugar, 125 grams butter
Whisk together the egg and milk. Add the milk, butter, and sweetened condensed milk to the flour. Mix with a wooden spoon until a soft dough has formed. Knead the dough a few times in the bowl, or really mix a few times after all the ingredients are combined with the wooden spoon to allow for some gluten formation.
125 grams milk, 1 egg, 395 grams sweetened condensed milk
Press the dough into the prepared baking dish.
Bake the rusks for 45 to 60 minutes or until a cake tester comes our clean from the centre of the rusks. The rusks will have doubled in size during the baking process.
Allow the rusk to cool for 10-15 minutes before turning it out onto a wire cooling rack to cool completely. Once completely cooled, slice the rusks using a serrated knife. Place the rusks on a baking sheet, in a single layer, and set your oven to a maximum of 110°C/230F (cool oven), preferably with the fan on to dry the rusks out. This process can take anywhere from 3 to 6 hours, usually dependent on the thickness of your rusks.
When the rusks are dry, store them in an airtight container, at room temperature, for up to a month. Serve with a hot cup or tea or coffee for dunking.