Preheat the oven to 180 °C.
Grease two loaf tins.
In a large bowl, combine all the dry ingredients except the sugar.
Add the sultanas and grated apple and lift the mixture with a wooden spoon a few times to incorporate the air.
Heat the margarine with the sugar and when melted, remove from the heat.
Mix the yoghurt, egg and vanilla essence and add to the margarine mixture.
Using a wooden spoon, mix the yoghurt mixture with the dry ingredients until just blended.
Add a little low-fat milk if the mixture is too stiff. (The dough should not be runny, but soft enough to spoon into the tins.)
Divide the mixture between the two loaf tins and bake for 30 minutes, reduce the heat to 150 °C and bake for another 45 minutes or until done.
Turn out of the pans and leave to cool.
Cut into fingers using a serrated knife and dry at 100 °C for two to three hours or until completely dry.
Nutritional value per rusk
Fat 2 g; Energy 349 kJ.