Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking dish.
Bring a large pot of lightly salted water to a boil. Add penne pasta, and cook for 8 to 10 minutes, or until al dente. Drain.
In a medium saucepan, melt butter over medium heat. Stir in flour, and cook for 1 minute. Gradually whisk in milk until smooth. Cook, stirring constantly until mixture thickens. Remove from heat and stir in Parmesan cheese and garlic powder.
Combine cooked pasta with sauce, and mix well. Pour into prepared baking dish.
Bake for 20 minutes in the preheated oven, or until golden brown on top.