Spray the insides of a 3-quart slow cooker with cooking spray. Mix the flour, white sugar, 2 tablespoons of the cocoa, baking powder, and salt in a medium bowl. Stir in the milk, oil, and vanilla until smooth. Stir in the walnuts. Spread this batter evenly in the slow cooker. Mix the brown sugar and ¼ cup cocoa in a small bowl. Stir in the hot water until smooth. Then pour this evenly over the batter in the slow cooker. Cover and cook on HIGH for 2- 2 ½ hours or until a toothpick inserted into the center comes out clean.
Turn cooker off and let the cake stand, uncovered, for 30-40 minutes before serving. Spoon the warm cake into serving dishes with some of the sauce on top.
NOTE: If you have a new slow cooker, they cook hotter than the older ones, so watch the cake so it doesn’t dry out or burn. Serve with ice cream for a sundae.