1. Weigh or lightly spoon flour into dry measuring cups, and level with a knife. Combine flour and 3/4 teaspoon salt in a large bowl, stirring with a whisk. Combine canola oil, 1/4 cup water, 1 tablespoon vinegar, and egg in a medium bowl. Gradually add oil mixture to flour mixture, stirring just until moist. Knead lightly until smooth. Shape dough into a ball, and wrap in plastic wrap. Chill for 1 hour.
2. Preheat broiler.
3. Place poblano on a foil-lined baking sheet; broil 8 minutes or until blackened, turning after 6 minutes. Place in a paper bag; close tightly. Let stand 15 minutes. Peel chile; cut in half lengthwise. Discard seeds and membranes. Finely chop.
4. Preheat oven to 400°.
5. Cook the cumin seeds in a large saucepan over medium heat 1 minute or until toasted, stirring constantly. Place cumin in a clean spice or coffee grinder; process until ground. Combine cumin, poblano, sweet potatoes, and next 5 ingredients (through 1/2 teaspoon salt) in a large bowl; mash with a fork until almost smooth.
6. Divide dough into 10 equal portions, shaping each into a ball. Roll each dough portion into a (5-inch) circle on a lightly floured surface. Working with 1 portion at a time (cover remaining dough to keep from drying), spoon 3 level tablespoons poblano mixture into center of each circle. Moisten edges of dough with egg white; fold dough over filling. Press edges together to seal. Place empanadas on a large baking sheet coated with cooking spray. Cut 3 diagonal slits across top of each empanada. Bake at 400° for 16 minutes or until lightly browned.