Preheat the oven to 200°C/400°F. Line a baking tray with baking paper.
In a large bowl, mix together the flour, grated Parmesan, thyme, salt and pepper.
Add the buttermilk and pumpkin purée to the dry ingredients. Using a knife or spatula, lightly mix until just combined. The dough should be soft but not sticky.
Turn the mixture onto a lightly floured surface and knead 2-3 times very gently to bring it together.
Press or roll the dough to about 4-5cm thick. Using a 5-6cm round cutter, cut out scones by pressing straight down without twisting. Re-form scraps gently and cut more scones.
Place scones on the prepared baking tray, leaving about 2cm between each.
Mix the egg yolk with 1 tablespoon of buttermilk and brush it over the tops of the scones.
Sprinkle with seeds if using.
Bake for 20-25 minutes until golden brown, and the scones sound hollow when tapped on the bottom.