Sift together the flour, salt and baking powder. Rub in the cold butter until pea-sized pieces remain; do not overmix.
Add the cold egg, buttermilk and shredded cheddar. Mix just until combined.
Gather the dough on a floured work surface and roll to 1/2–3/4 inch (1.5–2 cm) thick. Dip a 2.5-inch (6.5 cm) cutter in flour and cut rounds.
Place on a parchment-lined baking tray and chill for 30 minutes.
Brush the tops with beaten egg and sprinkle with extra cheddar. Bake on the middle rack of a preheated oven at 425°F (220°C) for 25–30 minutes, then cool on a rack.
Video
Notes
The preparation field retains the existing estimate; the video confirms the specific cooking or resting stages described above.