Spray the inside of a 2 ½ quart to 5 quart slow cooker with non stick spray.
In a medium bowl, mix the flour and baking powder together; set aside.
In a large microwave safe bowl, melt the butter and chocolate in the microwave in 30 second increments, mixing between each increment, cooking until melted (shouldn't be more than 2 minutes).
Adding to the butter mixture, whisk in 2/3 cup of sugar, 3 TBS of cocoa, vanilla, salt, milk, and egg yolk. Add the flour mixture and stir until thoroughly mixed. Pour the batter into the slow cooker and spread it evenly.
In a medium bowl, whisk together the 1/3 cup sugar, 1/3 cup brown sugar, 1/3 cup cocoa and hot water until completely mixed together. Pour the mixture over the batter in the slow cooker. - DO NOT MIX.
Cover and cook for 2-3 hours on HIGH, checking at the 2 hour mark. The cake should look done, and the sides of the cake will be away from the sides of the slow cooker. The larger your slow cooker, the shorter the cooking time will be. *I cooked in a small 2.5 quart slow cooker and it took exactly 2 hours.
Remove the top, be careful not to drip water from the lid onto the cake, and allow to cool for 30 minutes. Serve topped with ice cream or whipped cream.