1. Prepare the Dough

1. Preheat the Oven: Preheat your oven to 220°C (425°F) and line a baking tray with parchment paper.
2. Mix the Dry Ingredients: In a large mixing bowl, sift together the self-raising flour, baking powder, mustard powder, and a pinch of salt.
3. Rub in the Butter: Add the cold, cubed butter to the flour mixture. Using your fingertips, rub the butter into the flour until the mixture resembles breadcrumbs.
4. Add the Cheese: Stir in the grated cheddar cheese, leaving a little aside for topping the scones.

2. Form the Scones

1. Add the Milk: Gradually add the milk, mixing with a fork until a soft dough forms. Be careful not to overwork the dough.
2. Roll Out the Dough: On a lightly floured surface, gently roll out the dough to about 2-3 cm thick.
3. Cut the Scones: Use a round cutter or the rim of a glass to cut out rounds of dough. Place the scones on the prepared baking tray.
4. Brush with Egg and Top with Cheese: If using, brush the tops of the scones with the beaten egg and sprinkle with the reserved grated cheese.

3. Bake the Scones

1. Bake the Scones: Bake in the preheated oven for 12-15 minutes, or until the scones are golden brown and have risen.
2. Cool and Serve: Transfer the scones to a cooling rack and allow them to cool slightly before serving warm.