Spray the bowl of a 6-quart slow cooker with nonstick cooking spray and set aside.
In a large mixing bowl, combine the chocolate fudge cake mix, 1 cup milk, canola oil, and eggs. Use an electric hand mixer to blend until smooth, scraping the bowl sides as needed.
Pour the cake batter into the slow cooker and spread evenly.
In a separate bowl, whisk together the instant chocolate pudding mix and 2 cups cold milk until thickened and combined.
Slowly pour the pudding mixture over the cake batter. Do not stir.
Place a clean dish towel over the slow cooker bowl, then secure the lid ensuring the towel does not touch the heating element.
Cook on LOW for 3 to 3 1/2 hours until edges are set but center remains gooey.
Turn off the slow cooker, remove lid and towel, and let the cake cool inside for 30 to 45 minutes to set.
Spoon warm lava cake into bowls and serve with vanilla ice cream if desired.