Put the egg yolks in a bowl with 1/4 cup 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 in the orange juice and mix well.
Pour the mixture into a non-stick saucepan and cook gently over 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.
Add a pinch of salt to the eggwhites in a clean bowl and whisk into 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 some sprigs of fresh mint.