Preheat the oven to 180°C/Fan 160°C/Gas 4. Grease six 175ml metal dariole or pudding moulds with butter.
Cut a small circle of greaseproof paper to line the base of each mould. Cut 6 large, thin slices from the centre of the clementines (2 or 3 from each). Spoon a little golden syrup onto each paper disc and lay a clementine slice on top.
In a large bowl, beat the butter and sugar together, using an electric whisk, until smooth and creamy. Add the eggs, one at a time, beating after each addition. Add the remaining ingredients and beat until smooth. Spoon the mixture evenly into the dariole moulds. Stand them in a deep roasting tin.
Pour enough boiling water into the roasting tin to come halfway up the sides of the moulds. Cover the tin with a sheet of baking paper, then foil. Secure to the edges of the tin then place on the middle shelf of the oven to cook for 25 minutes.
Meanwhile, for the clementine syrup, heat the clementine juice and golden syrup together in a small pan, stir it and simmer for about 10-15 minutes until it turns syrupy.
Carefully remove the tray from the oven, remove the foil and baking paper and lift out the puddings. Loosen the edges with a palette knife and pop the puddings out onto warm serving plates. Spoon over some of the clementine syrup and serve with custard.