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Preheat the oven to 356°F/180°C. Grease and line the base and sides of an 8''X8'' (20X20cm) square cake pan with baking paper.
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Place the butter and chocolate in a microwave safe bowl. Microwave at 20% power for 15 seconds; remove the bowl from the microwave and stir. Repeat process until chocolate has melted and is smooth.
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Stir in sugar, egg, bananas, and vanilla. Sift over the flour and baking powder, stir to combine, then fold in the nuts and dark chocolate chips.
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Pour the batter into the pan and bake for 35 minutes or until golden. Cool the brownies completely in the pan.
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Melt chocolate. Cut the brownies into squares, sprinkle with turbinado sugar and drizzle with melted chocolate to serve.