Preheat oven to 200°C/180°C fan.
In a large mixing bowl, combine flour, salt, baking powder and mustard powder if using.
Rub the cold cubed butter into the flour mixture between your fingertips. You want most of it to resemble breadcrumbs, with a few pea-sized lumps of butter to help create the flaky layers later on.
Mix through your chopped fresh thyme and grated cheese.
Add the milk and combine until a shaggy dough forms.
Tip onto a clean work surface and mould into a rough rentangle. Fold the bottom third up and the top third down over that like a letter. This helps with the flaky layers. This is all very rough. Don't worry.
Flatten the dough until roughly 5cm thick. Either cut into squares or use a cutter like I have. If using a cutter, be sure to press straight down and don't twist side to side. This helps the scones to rise straight up and not become lopsided.
Arrange the scones on a baking sheet. Brush the top with milk and grind fresh black pepper over the top and sprinkle with grated Parmesan.
Bake for 20-25 minutes until golden brown all over. Best enjoyed on the day.