Cheddar Cheese Biscuits

2 cups all-purpose flour
2 tsps baking powder
1 tsp salt
1/2 tsp baking soda
1/2 tsp dry mustard
1 pinch cayenne pepper
2 tbsps unsalted butter (cut into pieces
1/4 stick)
1/4 tbsps gruyere cheese (grated, 3 1/2
oz)
5/8 cup buttermilk
1 egg
1 egg yolk (beaten with 1 t water glaze)
2 tbsps gruyere cheese (grated)
2 tbsps freshly grated parmesan (cheese)

Preheat oven to 450 degrees.
Butter baking sheet.
Sift flour, baking powder,salt, baking soda,mustard and cayenne into medium bowl.
Cut in butter until mixture forms coarse meal; mix in 14 tablespoons gruyere and make well in center.
Add 1/2 cup buttermilk and egg to well and blend with fork.
Mix into dry ingredients until just blended, adding more buttermilk to make sticky but workable dough.
Pat dough out on lightly floured surface to thickness of 3/4 inch.
Cut out 1 3/4-inch rounds, using fluted cutter; transfer to baking sheet.
Gather scraps, pat out again and cut additional rounds.
Lightly brush tops with glaze.
Combine 2 tablespoons Gruyere and Parmesan; sprinkle over biscuits.
Bake until light golden brown, about 14 minutes.
Serve hot with butter, if desired.