1. Preheat the oven to 350°F.

2. Line an 8-inch square baking pan.

3. Mix the cocoa powder and sugar with the milk. Add the bananas, brown sugar, and vanilla extract.

4. Beat the egg whites lightly with a fork. Add the chocolate mixture and continue to beat well. Sift the flour over the mixture and incorporate it with the oats. Pour into the prepared pan.

5. Bake for 40 minutes or until firm. Let cool in the pan for 10 minutes, then unmold onto a rack.

6. Cut into squares and sprinkle lightly with icing sugar before serving.