Put the egg yolks in a bowl with 50g of the sugar and whisk using an electric hand mixer until the mixture is pale and creamy. Blend in the cornflour and lime zest.
Gradually pour the orange juice into the egg yolk mixture and mix well.
Pour the mixture into a non-stick saucepan and cook gently on a low heat, stirring constantly with a wooden spoon, until the sauce begins to bubble and thicken. Remove the pan from the heat and add the liqueur, if using. Leave to cool for 30 minutes, stirring occasionally to prevent a skin forming.
Whisk the egg whites, with the pinch of salt, in a clean, grease-free bowl to form soft peaks. Gradually sprinkle in the remaining sugar and continue whisking until the meringue is glossy.
Fold the meringue mixture into the cooled orange sauce. Divide among four dessert glasses and chill in the fridge for at least 1 hour or until ready to serve.
Decorate with strips of orange and lime zest, and fresh mint sprigs.
To cool the sauce quickly at the end of step 3, plunge the base of the saucepan into a bowl of cold water.