Whisk the soy sauce, ketchup, garlic, brown sweetener, onion powder, sesame oil, and red pepper flakes in a small bowl, then set the mixture aside. Stir until the sweetener is fully incorporated and the sauce looks even in color.

Heat a nonstick skillet over medium-high heat and add the chicken pieces in a single layer. Cook for 5 to 8 minutes, turning as needed, until lightly browned and cooked through to 165 °F. The chicken will shift from releasing moisture to a more active sizzle as the surface begins to brown.

Add the prepared sauce to the cooked chicken, lower the heat to medium-low, and simmer for about 5 minutes while stirring to coat. The bubbles will become slower and glossier as the sauce thickens around the chicken; finish with sesame seeds.