To make the syrup, put the lime zest and juice, mint and sugar, into a small pan and heat gently until the sugar dissolves. Bubble for 1-2 minutes or until the mixture becomes a little syrupy. Cool and store in a sterilised jar. This will make more than you need but will keep, chilled, for a week.
To make the sour, put all of the ingredients in a cocktail shaker with ice and shake well until the outside of the shaker feels very cold. Strain into a chilled coupe glass and garnish with a lime wheel and a sprig of mint.