Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and garlic powder.
Cut the cold butter into the flour mixture until it resembles coarse crumbs.
Pour in the buttermilk and gently mix until a soft dough forms.
Turn the dough onto a lightly floured surface and pat it into a 1-inch thick rectangle.
Fold the dough over itself 3 times to create flaky layers.
Cut out 8 biscuits using a biscuit cutter.
Place the biscuits on the prepared baking sheet and bake for 15 to 18 minutes, or until golden brown.
Allow the biscuits to cool for 5 minutes.
Slice each biscuit in half.
Spread a thin layer of Dijon mustard on the inside if using.
Layer the ham, cheddar cheese, and Swiss cheese inside each biscuit.
Place the filled biscuits back on the baking sheet.
Bake for 3 to 5 minutes, just until the cheese is melted.
Brush the tops with melted butter and sprinkle with parsley or everything bagel seasoning if desired.
Serve warm.