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Heat oven to 180C/fan 160C/gas 4. Grease a 20cm springform cake tin or line a similar-size cake tin with baking paper. Using an electric mixer, combine 200g butter and 200g sugar in a large bowl. Beat until light and creamy, about 2 mins, then add the eggs, one at a time, beating well after you add each one. Tip in the flour and baking powder, mix for a few secs, then add the orange juice, zest of 2 oranges and poppy seeds. Stir everything together, then pour into the cake tin. Bake for 50 mins, then remove from the oven. Cool completely in the tin.
Meanwhile, make the candied orange zest by combining the remaining zest (finely sliced), 25g sugar and 100ml water in a small pan. Bring to the boil and cook for 5 mins until thick. Remove the zest with a slotted spoon onto a sheet of non-stick baking paper.
For the sauce, heat the cream in a small pan until nearly boiling. Put the chocolate and remaining butter into a bowl, then pour the hot cream over it and leave for 1 min to melt. Stir together until smooth, then let it sit until the cake is cool (if it hardens, just reheat gently). Drizzle the sauce over the cake, sprinkle with the candied zest and serve.