Place the chicken and the bacon on a plate and freeze to chill for 20 to 30 minutes.

Meanwhile, place the bread, chili powder, cayenne, and salt in the bowl of a food processor and pulse until you have fine bread crumbs. Divide the bread crumbs between two medium bowls. Set one bowl aside and stir the milk into the bread crumbs in the other bowl.

Add half the chilled chicken and all of the bacon to the food processor and pulse to combine into a rough mixture. Add the milk-and-bread-crumb mixture to the food processor along with the egg, onion, garlic, and Parmesan and process until very well combined (the mixture will be almost creamy). Add the remaining chilled chicken cubes to the food processor and pulse just to combine. The mixture will be semi-smooth with small chunks of chicken. Using a 1-inch small scoop, divide the mixture into about 30 small meatballs. Place the meatballs on a parchment paper-lined rimmed baking sheet and freeze for 15 minutes. Remove the pan from the freezer and roll the meatballs through the reserved bread crumbs to lightly and evenly coat the surface. Return the meatballs to the parchment-lined baking sheet.

Adjust an oven rack to the upper-middle position and preheat the broiler to high. Drizzle the meatballs with the oil and broil until they're cooked through, 5 to 6 minutes. Remove from the oven, sprinkle with Parmesan and the parsley (if using), rest for 5 minutes, and serve.