1. Preheat oven to 350°.
2. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, potato starch, cornstarch, and xanthan gum in a medium bowl; stir with a whisk.
3. Combine butter and chocolate in a large microwave-safe bowl. Microwave at HIGH 1 minute or until melted, stirring after 30 seconds. Add canola oil, stirring until smooth. Add sugar, vanilla, and eggs, and stir until well blended. Gradually add flour mixture, stirring until well blended. Spread batter into a 9-inch square metal baking pan coated with cooking spray. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack.