Preheat the oven to 190℃ conventional/170℃ fan-forced.
Line a baking tray.
In a large bowl, combine the flour, muesli, oatbran, sugar, baking powder, bicarbonate of soda,cream of tartar and salt. Mix well. Add the melted butter and buttermilk and, with a wooden spoon, stir for a few minutes until well combined.
Shape the mixture into 32 ovals using a dessert spoon and your hand. Place side by side, just touching, on the prepared tray.
Bake for 35 minutes or until golden.
Remove from the oven and allow to cool for 20 minutes.
Reduce the temperature to 100℃ conventional/80 ℃ fan-forced.
Separate and space out the rusks on the tray and bake for 6 hours or until golden and dry.
Allow to cool and store in an airtight container for many weeks.