For the best, flakiest biscuits, you need to keep your ingredients ice cold. Place the cubed butter and the buttermilk in the freezer 10-15 minutes before you begin.
Preheat your oven to 220℃ (200℃ fan/425℉).
In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, sugar, garlic powder, and salt.
Remove the butter from the freezer. Use a pastry cutter or your fingertips to work it into the flour until the mixture resembles course crumbs, and only small pieces of butter remain intact (no larger than the size of a pea).
Add the cheese and chives, and mix gently with a spatula or wooden spoon until combined.
Make a well in the centre of the flour mixture, and pour in the ice-cold buttermilk. Mix just until the dough starts to come together — it will be crumbly, with wet patches in places and loose crumbs at the bottom of the bowl.
Scrape the dough (crumbs and all) out onto a lightly floured surface. Use your hands to bring it together and gently press it into a rough rectangle about 3 cm (1 inch) thick. If the dough feels sticky, dust your hands or the surface of the dough with a little more flour. Fold the dough into thirds, like a letter, then rotate it 90 degrees. Gently press the dough back into a rectangle, and repeat this process 3 more times, incorporating any loose crumbs as you go.
Use a rolling pin to gently roll the biscuit dough out to an even thickness of 2 cm (3/4 inch). Cut the dough using a floured cookie cutter or biscuit cutter, pressing straight down. Don't twist the cutter — twisting seals the edges and prevents the biscuits from rising nice and tall. Carefully press the craps back together, and cut more biscuits. You shouldn't need to re-roll the dough more than once to get 6-8 biscuits.
Place the biscuits on a lined baking tray, arranging them so that they are just touching. If baking in a cast iron skillet, you'll need to place the biscuits on a plate or tray first for chilling, then transfer to the skillet right before baking.
Place the biscuits in the freezer for 15 to 20 minutes to firm up the butter.
Brush the tops of the biscuits with buttermilk or an egg wash. Bake on the middle rack for 12-15 minutes, or until golden brown.
These are best served warm! Brush the tops of the biscuits with melted butter and finish with a sprinkle of chopped chives. See notes below for storage and freezer instructions.