Line a baking tray with baking paper. Sift the flour and rice flour into a medium mixing bowl. Add the sugar and mix.
Cut butter into pieces and rub into the flour with your fingertips until the mixture begins to bind together. Knead into soft dough.
Roll the dough into an 20cm round (or for exact round, mould it in an 20cm cake tin). Place on baking tray. Using a fork, prick top and make tine marks along edge. Using a table knife, score top with wedge marks. (This is where it will break when cooled.)
Bake 45 minutes or until pale golden in colour. Sprinkle a little caster sugar over top and cool on baking tray. Cut into wedges. Keeps for weeks in airtight tin.