Prepare the syrup the day before. Combine all the ingredients in a saucepan and heat gently while dissolving the sugar, then bring to a boil for 5 minutes. Set aside to cool. Leave the spices and peel in the syrup and, when sufficiently cold, strain and pour into a glass bowl to refrigerate overnight. Shortly before use, place it in the freezer to get really cold.
For the koeksisters, keep 2 slotted spoons ready; one for lifting them out of the hot fat and another for putting them into and removing them from the syrup. Also, have a baking tray lined with a paper towel and another with a cake rack to drain the koeksisters after dipping them in syrup. Fill a large bowl with ice in which to stand the bowl with the strained syrup.
Sift the flour, baking powder, mixed spice, and salt into the bowl of a food processor. Add the butter and pulse until the mixture resembles a coarse meal. Add the milk and buttermilk and pulse until combined. Turn out onto a floured board and knead gently until the mixture forms a soft dough. Divide it into 2, place it in a bowl, and cover it with a damp tea towel. Leave it for 30 minutes to rest.
Pat one of the balls of dough out on a floured board into a rectangular shape about
2.5 cm thick. Using a floured rolling pin, roll it out into a rectangle 20 x 10 cm and 1 cm thick. Cut the rectangle into 4 strips across the width and 6 strips across the length (for 12 koeksisters). Cut each into 3 strips, leaving them joined across the top. Plait them, seal the ends with water and tuck the ends underneath. Place them on a baking tray and cover them with a damp tea towel while you repeat the process with the other ball of dough.
In a deep, heavy saucepan heat the oil to 180 °C and fry the koeksisters 5 at a time, turning them with a slotted spoon to brown evenly on both sides. Remove from the oil and drain on a paper towel for a short while. While still hot, drop them into the syrup for about 1 minute, remove with a slotted spoon, drain for a moment and place them on the cake rack.
Serve them warm (when they are at their best) or at room temperature.