Put the popping corn in a lidded bowl and microwave on High for 21⁄2-3 mins.
Tip into a big bowl and repeat four more times. Can be made and kept in an airtight container up to a day ahead. Meanwhile, melt the butter in a pan, then tip in the sugar. Heat gently until the sugar has dissolved. Pour the caramel over the popcorn and stir to coat. Cool, then serve in plastic cups.