In a medium heavy-bottomed saucepan over medium heat, whisk together the flour and salt until no lumps remain, and the flour is warm; 1 minute.
Slowly stream in the milk while whisking constantly to create a smooth mixture. Continue to whisk and cook the mixture until it begins to simmer. Once it begins to bubble, cook and whisk for a minute until mixture is thick. Turn off heat and whisk in the Redpath® Golden Yellow Sugar until sugar is dissolved and mixture is well combined.
Transfer and spread the mixture into a medium shallow dish. Place a piece of parchment paper or plastic wrap directly onto the surface of the mixture. The parchment paper helps to prevent a skin from forming as it cools. Cool completely or to room temperature (see Chef’s Tips).
In the bowl of a stand mixer with the paddle attachment, cream butter on medium speed until smooth. Increase speed to medium-high and beat until pale and fluffy; about 3 to 5 minutes.
Reduce the speed to medium and gradually beat in the room temperature flour mixture about two tablespoons at a time, waiting until fully incorporated before adding the next portion.
Once all the flour mixture has been incorporated, scrape down the sides of the bowl and paddle attachment with a rubber spatula. Switch to the whisk attachment and add in the lime zest, chopped mint, and vanilla extract. Mix on low speed to incorporate the additions, then increase speed to medium-high and beat until smooth and fluffy; about 3 minutes.
Transfer the frosting to a piping bag fitted with a star or round tip (see articles “Simple Decorating Techniques Using an Open or Closed Star Tip” & “Simple Decorating Techniques Using a Plain or Round Piping Tip”).
Decorate completely cooled cupcakes with the frosting using the piping bag. Alternatively, simply dollop some frosting onto the top of each cupcake and use an offset spatula or spoon and flatten or create a slight divot by making a circular motion in the centre of the frosting, pushing the frosting to the edges of the cupcake. If desired, garnish with fresh mint leaves, lime, and/or lime zest.