Place the dry ingredients (flour, baking powder, salt) in a large bowl.
Cut the super-cold butter into brunoise-size pieces and add to the dry ingredients. Using your hands mix the butter into the dry ingredients until the butter is evenly distributed, and make sure there are no large pieces.
Add the cheddar and chives, and again, evenly distribute by hand.
Pour the cold buttermilk over the dry ingredient mixture and mix by hand just until the buttermilk is incorporated. Do not over mix, as the dough should be very delicate and just coming together unlike bread dough.
Roll the biscuit dough out on a floured surface to one-inch thickness. Cut into two-inch squares.
Bake at 425 degrees until lightly brown.
Cut biscuits in half, lightly spread the mustard on one side and fill with 1 ½ ounces of ham. Serve warm or room temperature!
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