Mix together in a bowl all the dry ingredients: the flour, the baking powder, the mustard powder, the sugar, and the salt. Add a dash or two of pepper, if desired. Rub the butter in with your hands or a pastry cutter.

Stir in both cheeses (holding back a small handful of the grated cheddar), then use just enough of the buttermilk to make a dough that holds together without being wet or sticky.

Roll or spread the dough on a floured surface out to about 1/2 inch to 3/4 inch (1.5 to 2cm.) Cut out rounds about 2 inches wide (5 cm.)

Put on ungreased cookie sheet.

Mix together the 1 tablespoon of buttermilk with the mustard, and brush this over top the biscuits. Sprinkle the reserved cheddar over top. Bake for 12 to 15 minutes; they should be well-risen and golden brown. Serve warm or room temperature.