Cut chicken into 1-inch cubes. Pat completely dry with paper towels. Toss with garlic powder, smoked paprika, onion powder, salt, and pepper until evenly coated.
Heat olive oil in a large heavy skillet over medium-high heat until shimmering and close to smoking.
Add chicken in a single layer — cook in batches if needed. Sear for 3 to 4 minutes undisturbed until deeply golden. Flip and cook 2 to 3 minutes more until cooked through to 165°F (74°C). Transfer to a plate.
Reduce heat to medium. Add butter to the same pan. Once melted, add minced garlic and cook for 60 to 90 seconds, stirring constantly, until fragrant and just lightly golden — do not burn.
Add lemon juice and red pepper flakes if using. Stir and scrape up any fond from the pan.
Return chicken to the pan and toss to coat in the garlic butter. Cook 1 minute until glazed and heated through.
Remove from heat. Add fresh parsley and toss through. Taste and adjust seasoning. Serve immediately over rice, pasta, or flatbread with lemon wedges and crusty bread.