Note:
I made a double batch of above to fit into these two oven roasting trays as per photo.
I made the batch double as I bake in my big gas oven where two roasting trays fits next to one another.

Spray large rectangular oven roasting tray (the type that comes with your oven) with non-stick or coat generously with tin glide. Set oven temperature to 175°C to preheat.
Sift the flour, salt into a large mixing bowl. Add sugar and mix well. Add the buttermilk, eggs, vanilla and melted margarine/butter and stir until all the flour is incorporated into the wet ingredients.

Spoon into the oven tray and spread evenly. Pour a tablespoon of oil onto your hand and pat the surface of the dough level. The oil prevents the dough from sticking to your hand.
Bake for 40 to 45 minutes depending on your oven. In my gas oven I baked for 60 minutes.

Allow the tray to cool completely then cut into fingers and dry out at 65°C on trays or cookie sheets and with a wooden spoon in oven door to keep it slightly ajar so moisture can escape.

Once the rusks are dried out, allow it to cool down completely in the oven before packing it into airtight containers.

Note:
I cut each finger in half horizontally so that its smaller and can dry quicker. This way my rusks dry out in 4 hours. I also switch the trays around halfway through the drying time.
I dry out on two shelfs (position 2 and 4 from top of oven).
NB: I dry out in electric oven.