1 1/2 tablespoons Aleppo pepper* or 2 teaspoons dried crushed red pepper plus 2 teaspoons Hungarian sweet paprika, plus additional Aleppo pepper or paprika for sprinkling 1 cup plain whole-milk Greek-style yogurt** (8 ounces) 3 tablespoons extra-virgin olive oil 2 tablespoons red wine vinegar 2 tablespoons tomato paste 2 teaspoons coarse kosher salt 1 teaspoon freshly ground black pepper 6 garlic cloves, peeled, flattened 2 unpeeled lemons; 1 thinly sliced into rounds, 1 cut into wedges for serving 2 1/4 pounds skinless boneless chicken (thighs and/or breast halves), cut into 1 1/4-inch cubes If using Aleppo pepper, place in large bowl and mix in 1 tablespoon warm water. Let stand until thick paste forms, about 5 minutes. If using dried crushed red pepper and paprika combination, place in large bowl and stir in 2 tablespoons warm water and let stand until paste forms, about 5 minutes. Add yogurt, olive oil, red wine vinegar, tomato paste, 2 teaspoons coarse salt, and 1 teaspoon black pepper to spice mixture in bowl; whisk to blend. Stir in garlic and lemon slices, then chicken. Cover and chill at least 1 hour. Do ahead Can be made 1 day ahead. Keep chilled. Prepare barbecue (medium-high heat). Thread chicken pieces on metal skewers, dividing equally. Discard marinade in bowl. Sprinkle each skewer with salt, pepper, and additional Aleppo pepper or paprika. Brush grill rack with oil. Grill chicken until golden brown and cooked through, turning skewers occasionally, 10 to 12 minutes total. Transfer skewers to platter. Surround with lemon wedges and serve.