Spray inside of 2- to 3 1/2-quart slow cooker with cooking spray. Mix flour, granulated sugar, 2 tablespoons cocoa, the baking powder and salt in medium bowl. Stir in milk, oil and vanilla until smooth. Stir in nuts. Spread batter evenly in slow cooker.

Mix brown sugar and 1/4 cup cocoa in small bowl. Stir in hot water until smooth. Pour evenly over batter in slow cooker.

Cover and cook on high heat setting 2 hours to 2 hours 30 minutes or until toothpick inserted in center comes out clean.

Turn off slow cooker. Let cake stand uncovered 30 to 40 minutes to cool slightly before serving. Spoon warm cake into dessert dishes. Spoon sauce over top.