Preheat the oven to 180C Fan/ Gas 5.
Sieve the flour into a bowl and rub in the butter until the mixture resembles breadcrumbs.
Whisk the sugar and eggs together until frothy and then add the buttermilk, mixing well.
Add the buttermilk liquid to the flour and mix to form a soft and slightly sticky dough.
Form the dough roughly into balls the size of an egg. Using an icecream scoop works really well.
Grease a 30 x18 cm baking tin and pack the dough balls closely together.
Bake in the oven for 40 -45 mins until golden in colour.
Remove from the oven and leave to cool a little.
Turn the oven down to 120/130c.
When cool enough to handle break each large rusk in half. They look more rustic if you can gently break or tear them in half rather than using a knife to cut them. Place these back on baking trays and return to the oven for another hour or until crisp and thoroughly dried.
Store in an airtight container when cold.