How to Make It
Microwave butter and chocolate chips in a microwave-safe bowl on high 1 minute. Stir mixture until melted and incorporated. (If needed, microwave an additional 15 seconds.) Stir in flour, milk, vanilla, baking powder, salt, egg yolk, 2/3 cup granulated sugar, and 3 tablespoons cocoa powder just until incorporated. Spread evenly in a 4-quart slow cooker coated with cooking spray.
Stir together hot water, brown sugar, and remaining 6 tablespoons cocoa powder and 1/3 cup granulated sugar until combined. Pour over batter in slow cooker. (Do not stir.) Cover and cook on HIGH until top is set and browned, about 2 hours. Turn slow cooker off. Let stand 30 minutes before serving. Top servings with Cinnamon Whipped Cream.
For Cinnamon Whipped Cream: Beat 1 cup heavy cream and ¼ tsp. ground cinnamon at medium-high speed with an electric mixer until foamy. Gradually add 3 Tbsp. powdered sugar, beating until soft peaks form.