Preheat the oven to 200°C (400°F). Line a baking tray with baking paper.
Steam the butternut in a basket over boiling water with the lid on for 10 to 15 minutes until completely soft. Set aside to cool and allow the steam to evaporate. Blend until smooth using a food processor or immersion blender, or mash well by hand. Allow to cool completely before using.
Mix the flour, both cheeses, chives, salt and pepper together in a large bowl.
Add the buttermilk and cooled butternut purée to the dry ingredients. Using a knife, mix lightly until the dough just comes together. Do not overwork it.
Turn the dough out onto a lightly floured surface and knead very briefly, just enough to bring it together. Pat the dough to approximately 4 to 5cm thick.
Dip a 5 to 6cm round cookie cutter in flour and cut straight down through the dough without twisting. Place the scones on the lined baking tray.
Whisk the egg yolk and buttermilk together and brush only the tops of the scones. Scatter over the seeds if using.
Bake for 20 to 25 minutes until golden brown. Serve warm with butter.