In a microwave safe bowl, heat butter and chocolate chips 1 minute. Stir mixture until melted and well blended. May need a bit more microwave time if not melted.
Stir in flour, milk, vanilla, baking powder, salt, egg yolk, 2/3 cup sugar and 3 tablespoons cocoa until well blended.
Spread evenly into a sprayed 4 quart slow cooker. Stir together hot water, brown sugar, remaining 6 tablespoons cocoa powder and ⅓ cup sugar until well combined. Pour over batter into the slow cooker
Secure a paper towel under the lid of the slow cooker and cook on high about 2 hours or until top is set and browned.
Turn slow cooker off and let stand 30 minutes before serving. Top with whipped cream or vanilla ice cream, if desired.