Preheat your slow cooker and prep with a coat of cooking spray. In a medium size bowl, whisk together flour and baking powder; set aside. Next in a large bowl, melt the butter and chocolate together in the microwave; mix well. Whisk in 2/3 cup of sugar, milk, egg yolk, salt, vanilla extract, and 3 tablespoons of cocoa into the large bowl. Now, add the flour mixture and stir until ingredients fully mixed. Pour batter into slow cooker distributing evenly. Meanwhile, in medium bowl mix together the remaining sugar, cocoa and hot water until the sugar dissolves. Pour the mixture into the slow cooker. Cover and cook on high for 1-2 hours depending on the size of crock-pot. You will know cake is ready when nearly all the cake is set and edges begin to pull away from the sides of the pot. Once done, turn off cooker and remove lid.
Let it cool for 30 mins before serving. Top with ice cream or whipped cream.