In a large bowl, combine the flour, sugar, ½ cup cocoa powder, baking soda, baking powder, salt, buttermilk, water, vegetable oil, and vanilla extract. Pour the batter into a 6- to 8-quart slow cooker.
Sprinkle the chocolate chips, brown sugar, and remaining ¼ cup cocoa powder over the top of the cake batter.
Pour the hot water over the cake. (Do not stir.)
Cover and cook on high for 3 hours.
Serve warm. Store in the refrigerator for up to 4 days.