Preheat oven to 500°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, masa harina, 1/4 teaspoon salt, and chili powder in a food processor; pulse 3 times. Add water and butter; pulse until mixture forms a loose ball. Remove from processor; knead until ball completely forms. Divide dough into 8 equal portions. Shape each dough portion into a ball; flatten each ball into a 3-inch circle on a lightly floured surface. Cover and chill 30 minutes. (Stack dough portions between pieces of wax paper.)
Place poblanos on a foil-lined baking sheet. Bake at 500° for 20 minutes or until brown and blistered, turning once. Place in a zip-top plastic bag; seal. Let stand 15 minutes. Peel poblanos; cut in half lengthwise. Discard seeds and membranes, and finely chop. Place in a medium bowl.
Reduce oven temperature to 425°.
Heat a nonstick skillet coated with cooking spray over medium-high heat. Add garlic; sauté 30 seconds. Add 1/2 teaspoon salt and corn; sauté 3 minutes. Add to poblanos; let stand 5 minutes. Stir in cheese.
Roll each dough portion into a (5-inch) circle. Working with 1 circle at a time (cover remaining circles with a damp towel to prevent drying), spoon 3 level tablespoons corn filling into center of each circle. Moisten edges of dough with water; fold dough over filling. Press edges together with a fork or fingers to seal. Place empanadas on a large baking sheet coated with cooking spray. Combine egg white and 1 tablespoon water. Lightly coat tops of empanadas with egg mixture. Pierce top of dough with a fork. Bake at 425° for 20 minutes or until lightly browned.
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