Cook 8 ounces elbow pasta according to package directions until al dente. Drain the pasta and return to the saucepan, covering with a lid to keep it warm while you make the sauce.
Add 2 tablespoons butter to a medium saucepan over low to medium heat. Once melted, stir in 1 teaspoon garlic powder and 1/2 teaspoon salt until dissolved and fragrant, about 30 seconds.
Add 2 tablespoons butter to a medium saucepan over low to medium heat. Once melted, stir in 1 teaspoon garlic powder and 1/2 teaspoon salt until dissolved and fragrant, about 30 seconds.
Pour the cornstarch mixture into the saucepan and bring to a gentle simmer, stirring constantly. Cook for 3-4 minutes until noticeably thickened.
Remove the pan from heat and stir in 4 ounces shredded white cheddar cheese until completely melted and smooth. The residual heat will melt it perfectly.
Remove the saucepan from heat and then stir in 1/2 cup Greek yogurt until completely smooth. This cooling step prevents the yogurt from curdling.
Pour the cheese sauce over the warm pasta and stir to coat completely. Cover and let rest for 10-20 minutes before serving. This resting time allows the pasta to absorb the sauce for maximum creaminess. Add a splash more milk if needed before serving.