Preheat oven to 180 C / Gas 4. Grease a 23cm square baking dish.
Melt chocolate chips in a microwave-safe bowl on low power in 15-second intervals, stirring after each melting, 1 to 3 minutes. Cool slightly, about 5 minutes.
Mix flour, sugar, baking powder and salt together in a bowl. Add melted chocolate, bananas, eggs and vanilla extract; beat with an electric mixer on medium speed until well-blended. Stir in pecans. Spread brownie mixture in the prepared baking dish.
Bake in the preheated oven until top is dry and edges have started to pull away from the sides of the pan, 30 to 35 minutes. Let cool, about 25 minutes.
Combine butter and cocoa powder in a saucepan over low heat; cook, stirring constantly, until smooth, about 3 minutes. Stir in icing sugar and milk. Add more milk, 1 tablespoon at a time, until icing is spreadable.
Spread icing over cooled brownies. Cut into squares.