Generously butter rings measuring 7 cm in diameter. Fill with the purée and place 1 whole egg yolk in the middle of each.
Close over with the purée. Cover and steam for 9 minutes at 90° (allow 1 or 2 minutes more if the oven is very full).
Set aside for 3 minutes. Unmould and pour a light Taleggio fondue over top. Add a little grated white truffle and garnish with squash chips.
Mix together the squash and potato purées, let cool and mix in 1 well-beaten egg white with a pinch of salt.