Preheat the oven to 180°C. Line a 12-cup muffin tin with cases.
In a large bowl, whisk together milk and vinegar. Set aside for 5 minutes. After 5 minutes, whisk in oil, egg, vanilla and salt.
Place cupcake mix in a large bowl and slowly fold in wet mix, until no lumps remain. Be careful not to overmix or cupcakes will be tough. The mixture will be quite liquidy.
Divide evenly between muffin cups and bake for 18-20 minutes, until a knife comes out clean. They will be quite pale. Allow to cool completely before decorating.
To make icing, place room temperature butter in a large bowl or bowl of your stand mixer. Use electric beaters to beat until pale, about 4 minutes. Add icing mix and milk to bowl and continue to beat a further 5 minutes, until light and fluffy.
Top cooled cupcakes with strawberry icing, using a spoon or piping bag.