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 squash just until tender but firm enough to hold shape, stirring occasionally, about 15 minutes. Transfer squash to bowl. Cool.
DO AHEAD: 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.
Separate rice grains. Add rice and butternut squash; simmer until heated through and liquid is absorbed, about 4 minutes.
Stir in 2 tablespoons butter and parsley. Season with salt and pepper.
Transfer to bowl and serve.
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