In a large mixing bowl, combine all-purpose flour, baking powder, and kosher salt. Cut in very cold unsalted butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Add mustard and ice water, mixing until a dough forms. Wrap the dough in plastic wrap and chill in the refrigerator for 1 hour to develop flavors and texture.

Preheat your oven to 400°F (200°C). On a floured surface, roll out the chilled dough into a large 16-inch square. Cut the dough into four equal 8-inch squares, ensuring even edges.

Lay the dough squares on a parchment-lined baking sheet. On one-half of each square, evenly layer the sliced deli ham and generous pieces of creamy brie. Sprinkle chopped chives, fresh cracked pepper, and a pinch of salt over the filling.

To seal your Ham and Brie Pop Tarts, brush the edges of the dough squares with egg wash. Fold the dough over the filling, creating a pocket. Crimp the edges with a fork to ensure they are well sealed.

Place the filled and sealed hand pies in your preheated oven and bake for 15 minutes. Rotate the baking sheets to ensure even baking and continue to bake for another 10-15 minutes, or until the crust turns golden brown.

Once baked to perfection, allow the Ham and Brie Pop Tarts to cool on the baking sheet for about 10 minutes. Serve warm or let them cool to room temperature.