Rinse rice in strainer under cold water; drain. Bring 6 cups water and 1 teaspoon coarse salt to boil in large saucepan. Add rice; bring to boil. Reduce heat; simmer uncovered until rice grains begin to split and are tender but still slightly chewy, about 45 minutes (be sure to test for doneness before draining -- some varieties take longer than others). Drain. Spread on rimmed baking sheet to cool. Transfer to bowl.
Preheat oven to 350°F. Oil rimmed baking sheet. Toss squash cubes and 3 tablespoons oil in medium bowl. Spread squash in single layer on prepared sheet; sprinkle with salt and pepper. Roast just until tender but firm enough to hold shape, stirring occasionally, about 15 minutes. Transfer squash to bowl. Cool. Can be made 1 day ahead. Cover and chill.
Melt 4 tablespoons butter in large skillet over medium heat. Add leeks and 3/4 cup water; simmer until leeks are tender, about 7 minutes. Add corn; simmer 2 minutes longer. Add rice and butternut squash; simmer until heated through and liquid is absorbed, about 4 min. Stir in 2 tbsp butter and parsley. Season with salt and pepper. Transfer to bowl and serve.
Adapted from Bon Appétit, November 2008, by Nancy Oakes and Pamela Mazzola