Line slow cooker with aluminum foil collar and lightly coat with vegetable oil spray. You will need an oval slow cooker for this recipe.
Whisk flour, ½ cup sugar, ¼ cup cocoa, cinnamon, baking powder, and salt together in large bowl.
In separate bowl, whisk together milk, melted butter, egg yolk, and vanilla. Stir milk mixture into flour mixture until just combined.
Fold in chocolate chips (batter will be stiff).
Then pour into the basin of a slow cooker generously coated with nonstick. Scrape batter into prepared slow cooker and spread to edges.
Sprinkle a thin layer of chocolate chips over it. Set aside.
Combine remaining ½ cup sugar and remaining ¼ cup cocoa in clean bowl and sprinkle evenly over top.
For an accurate measurement of boiling water, bring a full kettle of water to a boil and then measure out the desired amount. Slowly pour boiling water evenly over top. Do not stir.
Cover and cook until cake is puffed, top is lightly set, and center is gooey when pierced with toothpick, 1 to 2 hours on high.
Discard foil collar. Turn off slow cooker and let cake sit, covered, for 10 minutes before serving.
Serve in bowls topped with ice cream and toffee bits.