Spray the inside of a 2 – 3 1/2 quart slow cooker with cooking spray.
Mix flour, sugar, 2 TBSP cocoa, baking powder and salt in a medium bowl. Stir in milk, oil and vanilla until smooth. Stir in nuts. Spread batter evenly in slow cooker.
Mix brown sugar and ¼ cup cocoa in a small bowl. Stir in hot water until smooth. Pour evenly over the batter in the slow cooker.
Cover and cook on high heat setting for 2 -2 ½ hours or until tooth pick inserted comes out clean.
Turn off slow cooker and let stand uncovered 30-40 minutes to cool slightly and to let the sauce thicken before serving.
Spoon warm cake into dessert dishes with sauce on top. Serve with vanilla ice cream.