Lightly coat the inside of a 3-1/2- or 4-quart slow cooker with nonstick cooking spray; set aside.
For batter, in a medium bowl stir together the brownie mix, melted butter, eggs, and the 1/4 cup water until batter is nearly smooth. Spread brownie batter evenly in the bottom of the prepared cooker.
In another bowl combine the sugar and cocoa powder. Gradually stir the boiling water into the sugar-cocoa mixture. Pour evenly over batter in cooker.
Cover and cook on high-heat setting for 2 hours (center may appear moist but will set up upon standing). Remove liner from cooker, if possible, or turn off cooker. Let stand, uncovered, for 30 to 45 minutes to cool slightly before serving.
To serve, spoon warm cake into dessert dishes; spoon pudding over cake. If desired, top with a scoop of ice cream. Makes 8 servings.