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Preheat oven to 180C/ Gas 4 and line a 20cm pan.
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Melt the chocolate and butter together over a low heat.
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In a separate bowl, whisk the aqua faba with an electric whisk for a few minutes until frothy. Add the sugar in, a little at a time, and continue whisking until thick and glossy. This will take around 5 minutes.
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Pour the melted chocolate into the aqua faba mixture and stir gently.
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Add the flour and cocoa powder and fold in.
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Chop the shortbread into small chunks and add to the batter. Leave a few pieces aside for sprinkling on top.
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Bake for 25-30 minutes until the center is firm and not wobbly.
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Leave to cool in the pan and refrigerate for 2 hours to firm up.