Mix all seasoning spices into the ground chicken until evenly coated throughout.
Heat a 12-inch cast iron or stainless steel skillet over medium-high heat until hot. Add the seasoned chicken and break immediately into small crumbles with a wooden spoon. Cook 5 to 7 minutes, stirring every 90 seconds, until the meat is fully cooked with no pink remaining and lightly browned. Confirm 74°C (165°F) internal temperature.
Reduce heat to medium. Sprinkle flour evenly over the cooked meat. Stir continuously for 90 seconds to 2 minutes until the flour is absorbed into the fat and the raw flour smell cooks off. The mixture should look moist and coated, not dry.
Pour 480ml of room-temperature milk in a slow steady stream, whisking continuously. The mixture will thicken quickly; keep whisking until smooth. Stir over medium heat for 3 to 4 minutes until the gravy thickens and bubbles gently.
Add remaining milk gradually, stirring as it goes in. Cook on medium-low for 4 to 5 more minutes until the gravy coats the back of a spoon and holds a clear finger-drawn line for 3 to 4 seconds.
Remove from heat. Taste and season with extra salt and black pepper. Stir in the optional butter until melted. Serve immediately over split biscuits with an extra crack of black pepper over each serving.