Preheat the oven to 200 C (400 F): As the oven approaches 200 C you should feel the warmth building in the kitchen and a faint dry heat when you open the door. Preheating is essential because a hot oven gives an immediate oven spring, helping the scones rise and form a golden crust. If you skip this the scones may bake slowly and become dense rather than airy. One common mistake is setting the oven too low to 'save energy' which results in pale, underbaked scones, so allow full preheating time.
Steam the butternut in a metal basket or sieve over boiling water (lid on). Cook for 10 – 15 minutes until soft. Set aside to cool and dry off in the air. Blend in a food processor or with an immersion blender until smooth (you could also mash by hand): When you steam the butternut , you will notice a sweet, vegetal aroma as the cubes soften, and the steam will make the flesh tender. Cooling is important because hot puree adds unwanted moisture and can alter dough temperature, making the butter and flour behave differently. Blending yields a silky, uniform texture that mixes smoothly with dry ingredients, while hand mashing keeps a slightly rustic bit of texture. A typical mistake is not draining enough steam which leaves the puree watery, so spread it on a tray to dry for a few minutes if it seems wet.
Mix the flour, cheese, chives, salt and pepper in a bowl.: When you combine the self raising flour , Parmesan , Gruyere , chives , salt , and black pepper , the dry mixture should smell savory and a bit nutty from the cheeses. Rubbing the cheese lightly into the flour helps distribute it so pockets of flavor form evenly as you bake. The visual cue is a homogeneous pale mixture flecked with green and pale cheese granules. Overmixing at this stage can bruise the herbs and make the dough tough, so stir just to combine.
Add the buttermilk and butternut puree to the dry mixture and using a knife lightly mix it until it is just combined.: As you incorporate the buttermilk and the butternut purée, the bowl will emit a warm scent of squash and tang from the dairy, and the texture will move from powdery to shaggy. Using a knife to fold prevents overworking gluten, which is why you want the dough barely joined. If the mixture looks too wet, rest it briefly and the flour will hydrate; if it looks dry, add a teaspoon of buttermilk at a time. A frequent error is vigorous stirring which produces tough scones, so stop when the ingredients no longer look separate.
Turn the mixture out onto a lightly floured surface and knead slightly to form a dough.: The dough should feel soft and slightly tacky to touch, not sticky. Knead only to bring it together, pressing with the heels of your hands and folding a couple of times. You will hear a soft friction and feel the dough come together into a cohesive mass. Overkneading develops gluten and makes scones heavy, so keep your movements brief and confident. If you find it sticking, dust lightly with self raising flour , but avoid adding too much which dries the final product.
Press the dough down to about 4-5 cm and cut the scones using a 5 – 6 cm cookie cutter and place on a baking sheet lined with silicone or baking paper.: Pressing the dough creates even thickness so scones bake uniformly, and the height will influence the interior texture, yielding a cloudlike center when set correctly. Use a firm push with your palm to compress the dough to the recommended height and press straight down with the cutter to avoid sealing the edges. Arrange them with space between so air circulates and edges crisp. A common pitfall is twisting the cutter which seals the edges and prevents a good rise, so lift straight up after pressing.
Mix the egg yolk and extra buttermilk together and brush the tops of the scones (NOT the sides): The glaze of egg yolk and buttermilk gives a glossy, deep brown finish. Apply with a light brush stroke across the tops only, because wet sides can prevent proper rise and may stick to the baking paper. You will see the tops catch the light and look slightly wetter, which is the cue to stop. If you get glaze on the sides, blot gently with your finger to tidy edges before baking.
Sprinkle on the seeds, and bake in the oven for 20 – 25 minutes until golden brown.: As they bake, the kitchen fills with toasty, cheesy aromas and you may hear a soft crackle as the exterior sets. Look for a deep golden color and a firm top when pressed lightly. The internal temperature should feel warm and springy, not doughy. A common mistake is opening the oven repeatedly which drops temperature and may flatten the scones, so peek sparingly and rely on color more than exact minutes.