In a mixing bowl, mix together 1 cup brown sugar, flour, 3 Tbsp. cocoa, baking powder, and salt.
Stir in milk, butter and vanilla. Spread over bottom of slow cooker (batter will be really thick).
Mix together 3/4 cup brown sugar and 1/4 cup cocoa. Sprinkle over mixture in slow cooker.
Pour boiling water over the top. Do not stir.
Cover and cook on HIGH 2-3 hours, or until a toothpick inserted comes out clean.
Serve warm with vanilla ice cream. ENJOY!!