Boil the pasta in salted water until just tender. Reserve 1 cup pasta water, then drain without rinsing.
While the pasta cooks, heat olive oil in a deep skillet over medium-low. Add garlic and cook for 30 seconds without browning.
Whisk in broth and cream cheese. Keep the heat gentle and whisk until the cream cheese is smooth.
Reduce heat to low. Whisk in hot sauce, Greek yogurt, 3/4 cup cheddar, garlic powder and black pepper until combined; do not let the sauce boil.
Stir in cooked chicken and heat until the thickest pieces reach 165°F, adding reserved pasta water a few tablespoons at a time if the sauce tightens.
Fold in the drained pasta. Rest off heat for 5 minutes, loosen with more pasta water if needed, and top with remaining cheddar and scallions.
