1) Set a rack in the centre of the oven and preheat oven to 230C/Gas 8. Line a baking sheet with two layers of parchment paper.
2) In a large bowl, whisk together the flour, cornmeal, baking powder, bicarbonate of soda, sugar, and salt. Rub 30g of the butter into the flour mixture with your fingertips, until it is completely incorporated. Work the remaining butter into the flour until it's in even, pea-sizes pieces. Gently stir in the buttermilk to make a loose dough.
3) Turn the dough out onto a lightly floured work surface. Pat the dough into a 1cm thick rectangle. Fold the dough in thirds like a business letter. Pat the dough into a 13 by 20cm rectangle about 2cm thick. Use a 5 to 8cm round cutter to make 6 biscuits, and put biscuits on the prepared baking sheet. Press together the scraps of dough, cut 2 more biscuits, and place them on the baking sheet.
4) Bake until the tops are lightly browned, about 15 minutes. Cool slightly on a rack before serving.