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Pre-heat the oven to 175ÂșC.
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Grease and flour a loaf tin (dimensions 21.6 cm wide, 11.4 cm deep, 5.5 cm height).
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Whisk eggs, sugar and vanilla sugar until pale and foamy.
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Blend flour and baking powder together and fold into mixture alternately with melted butter.
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Then fold in the mashed banana.
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Put in a loaf tin and cook in the centre of the oven for about 50 minutes or until the cake springs back when prodded.
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Leave to cool for a few minutes and then turn out onto a wire rack to fully cool.
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Make the chocolate topping by melting the chocolate and butter in a small glass bowl over a pan of barely simmering water, stirring until melted.
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Scrape the chocolate out of the bowl and onto the cake with a spatula.
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Use a palette knife to spread the chocolate evenly over cake.
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Allow to set (we have never managed to wait this long though)!