Slice the artichokes into 1 cm (½ inch) thick pieces. Place in a saucepan over medium heat with the butter and 400 ml (14 fl oz) water. Season and cook for 30 minutes or until softened and the water is evaporated. Drain and place in a blender with the eggs and gellan. Blend until smooth. Pass through a sieve. Return to the pan, place over medium heat and cook, stirring continuously, until the purée reaches 85°C (185°F). Transfer to a metal bowl and chill over ice to firm up. Place in a piping bag fitted with a 1.5 cm (⅝ inch) plain nozzle. This can be made 1 day in advance and refrigerated. Bring to room temperature before serving.