Heat the oven to 180C/fan 160C/gas 4 and line a 12 hole bun tin with cupcake cases.
Sift the folour into a bowl and stir in the baking powder and salt.
In a seperate bowl, beat the butter and sugar together with an electric hand mixer, until light and fluffy. Add the eggs one at a time, mixing well after each addition. Add the vanilla extract, then the flour mixture and finally the lime zest and milk. Mix until well combined.
Divide the mixture between the cupcake cases, filling about two-thirds full.
Bake for 15 - 18 minutes, until well risen and a skewer inserted in the middle comes out clean. Cool in the tin for 5 minutes, then remove from the tin and leave to cool completely on a cooling rack.
For the icing, beat the butter with an electric hand mixer for 2 - 3 minutes until soft. Gradually add the icing sugar, beating well after each addition. Add the tequila, lime juice and salt, then beat on a medium speed for a further 2 - 3 minutes until light and fluffy.
Spoon the icing into a piping bag fitted with a 1.5cm round nozzle and pipe onto the cupcakes.
Mix the tequila and triple sec together. Cut off the tip of the end of twelve Bottle Toppers. Place the cut ends into the tequila and triple sec mixture and squeeze to fill. Insert the cut ends into the iced cupcakes. Sprinkle over the lime zest and garnish with lime slices.
When ready to eat, squeeze the top of the Bottle Toppers to infuse the cupcakes with tequila and triple sec.