Preheat oven to 350°. Combine semisweet chocolate, canola oil, and butter in a bowl. Microwave at HIGH 45 seconds, stirring occasionally. Stir in brown sugar, granulated sugar, and egg. Stir in flour, cocoa, baking soda, and salt. Stir in chopped milk chocolate. Scrape into a 10-inch cast-iron skillet. Bake at 350° for 19 minutes.
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