In a mixing bowl, beat the eggs with the salt and sugar about 5 minutes. Add the softened butter and mix. Stir in the juice and zest. Pour into 4 individual ramekins.
Place the ramekins in a deep baking tray and add hot water to halfway up the sides of the ramekins. Bake 25 minutes.
Remove from the water bath and chill ramekins in the fridge at least two hours. Garnish with mint leaves before serving.