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Heat the oven to 190C/170C fan/gas 5 and line a shallow 23cm square baking tin with baking parchment. Tip the beans, cocoa, coffee granules, xylitol, vanilla, eggs and yogurt into a large bowl. Blitz using a hand blender until smooth. Or, do this in a food processor. Add the baking powder, then blitz again.
Quickly tip the mixture into the tin and bake for 25-30 mins until set. Leave to cool in the tin for a few minutes, then lift out onto a wire rack to cool completely. Remove the baking parchment, then cut into 16 squares. Will keep chilled for up to three days.