Preheat the oven to 220°C, 425°F, or 200 Fan. Line a baking tray with parchment paper.
Mix together the self-rising flour, corn flour, baking powder, salt, and mustard powder (if using) in a large bowl. Grate or cube the butter and rub it into the flour until no large pieces of butter remain.
Combine the yoghurt and milk in a jug.
Add 200 grams of cheese to the flour mixture and stir to combine. Make a well in the middle of the mixture and pour in two thirds of the liquid. You might not end up needing all of it, so don’t pour it all in at once.
Mix together with a wooden spoon or your hands and add the rest of the liquid if needed. Only mix until all the dry ingredients are incorporated, be careful not to knead the dough. Place all the dough on a clean, lightly floured surface.
Roll the dough out until it reaches a thickness of 2 cm. Using a round, sharp cutter, press out circles of the dough in one rapid motion - don’t twist the cutter.
Place the scones on a lined tray and brush with milk. Distribute the remaining cheese on the scones and place in the oven for 14-16 minutes, or until golden brown.
Serve warm or allow to cool before storing in an airtight container. They will last 2-3 days at room temperature, or 3-5 days in the fridge stored with a kitchen towel to soak up condensation. You can also freeze them for up to 3 months.