Preheat the oven to 180˚C. Grease two baking trays and set aside.
Sift the flour, cream of tartar and salt together. Now add the sugar and stir until combined.
Using your fingertips rub the butter into the flour until it resembles breadcrumbs.
In a separate bowl, mix together 25ml of the buttermilk with the baking soda, stir until combined. Add this buttermilk to the flour mixture and mix until combined.
Beat the egg and the remaining buttermilk together. Keep 25ml of this mixture aside. (You will use this mixture to brush over the rusks before they are baked.)
Add the remainder of the egg mixture to the flour mixture and mixed until well combined.
Divide the dough into 4 even pieces. On a lightly floured surface, roll and shape each piece into a long 30cm log. Slightly flatten the tops of the logs.
Place the dough logs on the greased baking trays, ensuring enough space between them. This will help them bake evenly. Brush the reserved egg/buttermilk mixture over the tops of each log.
Bake for 20-30 minutes or until golden and baked throughout.
Remove from the oven and allow to cool for 5 minutes. In the meantime, reduce the oven heat to 70˚C.
Using a sharp knife, slice the logs into slices.
Slicing each slice +/- 2-3 cm thick. Place the slices back onto the baking tray. Leave enough space between them to allow airflow, this will help dry out the rusks.
Dry the rusks out for 3-4 hours or until completely dry.
Remove from the oven and allow the rusks to cool off completely before storing them in an airtight container.