1. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, sugar, cocoa, and salt, stirring with a whisk.
2. Combine canola oil and next 3 ingredients (through eggs), stirring with a whisk. Add to dry ingredients, stirring until blended. Stir in melted chocolate. Stir in walnuts.
3. Coat a 3-quart electric slow cooker with cooking spray. Pour batter into slow cooker. Cover and cook on LOW for 2 to 2 1/2 hours or until set around edges but still soft in the center. Turn off slow cooker. Let stand, covered, 30 minutes before serving. Sprinkle with powdered sugar, if desired.