Place the ketchup, soy sauce, vinegar, black pepper, and garlic powder in a resealable plastic bag, and add the chestnuts. Squeeze the bag several times to mix the ingredients, and squeeze air out of the bag. Seal the bag, and marinate the chestnuts overnight.
The next day, preheat oven to 400 degrees F (205 degrees C). Line a baking sheet with parchment paper.
Remove chestnuts from the marinade, and shake off excess marinade. Wrap each chestnut in half a bacon slice, and secure with a toothpick. Place brown sugar into a shallow bowl, and roll each wrapped chestnut in brown sugar. Place the appetizers onto the prepared baking sheet.
Bake chestnuts in the preheated oven until the bacon is crisp and the brown sugar is bubbling, about 20 minutes. Allow to cool before serving.
