Preheat your grill to medium-high heat (about 400°F) or preheat your oven to 400°F. Lay out four large sheets of heavy-duty aluminum foil (approximately 18x12 inches each) on a flat surface.

In a large bowl, combine the halved baby potatoes, sliced kielbasa, garlic powder, smoked paprika, salt, and black pepper. Toss until everything is evenly coated with the seasonings.

Divide the kielbasa and potato mixture evenly among the four foil sheets, piling it in the center of each sheet.

Dot each pile with 1 tablespoon of butter (cut into small pats) and drizzle or dollop approximately 3/4 tablespoon of spicy brown mustard over each portion.

Fold the foil over the filling, bringing the long sides up and rolling them down to seal tightly. Then fold and crimp both short ends to create a fully sealed packet with no gaps. For extra security, double-wrap each packet with a second sheet of foil.

GRILL METHOD: Place packets on the grill over medium-high heat. Cook for 20-25 minutes, flipping once halfway through at the 10-12 minute mark, until the potatoes are fork-tender and the kielbasa is caramelized.

OVEN METHOD: Place packets on a rimmed baking sheet and bake at 400°F for 30-35 minutes (no need to flip) until the potatoes are fully tender when pierced through the foil with a knife.

Remove packets from heat and let rest for 3-5 minutes before opening carefully — steam inside is very hot. Open away from your face, peeling back the foil slowly.

Garnish with fresh chopped parsley if desired and serve directly from the foil, or slide contents onto plates. Enjoy immediately.