Preheat the oven to 400°F.
In a medium bowl measure the flour, baking powder, baking soda, and salt. Whisk to combine. In a separate measuring bowl add the milk and vinegar. Set aside for the milk to curdle into buttermilk. Set out a large baking sheet with parchment paper.
Cut the cold butter into small 1/4″ cubes and add to the flour mixture. Working quickly, use your fingers to press/squish the butter into flat discs in the dough. Try to get all of the cubes if possible. It does not have to be 100% incorporated. Set aside in the fridge for a few minutes.
Thinly slice/mince the chives and shred the cheddar (shred a few extra tbsp for topping if desired). Add the chives and cheddar to the flour mixture and stir to combine.
Whisk one egg into the buttermilk mixture. Whisk the other egg in a small bowl to be used as egg wash. Make a well in the middle of the flour mixture before adding in the butttermilk/egg mixture. Stir with the fork until it begins to clump together, then dump on a clean counter.
Give the dough a few folds to incorporate all the stray dough, adding more milk or flour if necessary. Do NOT knead or overwork. It will be a bit sticky (not wet), use a bench scraper if needed.
Work the dough into a round disc that is ½” – ¾″ thick. Cut into 9 equal “pie” pieces or use a 2 ½″ round biscuit cutter (a wide mouth jar works too). Place the cut biscuits onto the prepared baking sheet and brush the top with egg wash. Top with extra cheese if desired or a pinch of flaky salt before placing in the preheated oven.
Bake for 18-20 minutes until lightly golden brown and fully cooked. Let them cool for a few minutes before tearing open and devouring. Enjoy!
