Make buttermilk biscuits: mix flours, baking powder and soda, salt, and pepper in mixing bowl. Cut 6 T. butter into small pieces and add to flour mixture. Cut in butter with pastry blender or 2 knives until mixture resembles fine meal. Melt remaining 4 T. butter and set aside. Make well in flour mixture, pour in buttermilk, and gradually work flour into buttermilk using fork.
Turn dough out onto lightly floured surface. Roll or pat evenly 1/2-inch thick. Using 2-1/2" biscuit cutter or rim of glass, cut out 12 biscuits. Brush both sides with melted butter and place on ungreased baking sheet.
Bake until golden, 10-12 minutes. Let cool on baking sheet at least 5 minutes before splitting.
Split warm biscuits in half. Spread bottom half with mustard and top with butter. Place 2 slices ham on each bottom half and cover with top. Serve warm.