01 - Preheat oven to 425°F. Line a baking sheet with parchment paper.
02 - In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, sea salt, and ground black pepper until evenly combined.
03 - Add the cold, cubed unsalted butter. Using a pastry cutter or fingertips, blend into the dry mixture until the consistency resembles coarse crumbs.
04 - Gently fold in shredded cheddar cheese, diced avocado, and chopped chives until distributed throughout the mixture.
05 - Pour in the cold buttermilk and stir just until the dough begins to hold together. Avoid overmixing.
06 - Transfer dough to a floured work surface. Pat or lightly roll into a 1-inch thick rectangle.
07 - Utilize a 2.5-inch round biscuit cutter to portion out biscuits. Transfer to the prepared baking sheet, gathering and reshaping dough scraps as necessary.
08 - Whisk together egg and milk in a small bowl. Lightly brush biscuit tops with the egg wash using a pastry brush.
09 - Bake for 16–18 minutes or until biscuit tops are golden brown.
10 - Allow biscuits to cool slightly before serving warm.