Preheat the oven to 220°C, fan 200°C, gas 7. Lightly grease a 23cm square cake tin.
Sift the flour and salt into a large bowl. Rub in the butter, using your fingertips, until the mixture resembles coarse breadcrumbs. Add the chopped rosemary and most of the olives, reserving a few to garnish.
Stir in the buttermilk and enough water to make a soft, sticky dough (about 125ml). Tip on to a lightly floured surface and knead until smooth. Press the dough into the tin, shaping to fit. Use a knife to cut the dough into 12 pieces. Brush the tops with any extra buttermilk and top with the remaining rosemary and olives.
Bake for about 30 minutes or until golden. Leave to stand in the tin for about 5 minutes before turning out.