01 - Preheat oven to 425°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together flour, baking powder, baking soda, sea salt, and black pepper.
03 - Add cold cubed butter and cut it into the dry mixture using a pastry cutter or fingertips until the mixture resembles coarse crumbs.
04 - Fold in shredded cheddar cheese, diced avocado, and chopped chives.
05 - Pour cold buttermilk into the mixture and gently mix until just combined. Avoid overworking the dough.
06 - Transfer dough onto a floured surface. Pat or roll to about 1 inch thick.
07 - Use a 2.5-inch round cutter to portion biscuits. Place onto prepared baking sheet, gathering scraps to re-cut as needed.
08 - Whisk the egg with milk, then brush over biscuit tops.
09 - Bake for 16 to 18 minutes, or until biscuits are golden brown.
10 - Allow biscuits to cool slightly and serve warm.