This is Stevia rebaudiana, otherwise known as The Sugar Plant or Candy Leaf. It is said that the leaf has 300 times the sweetness of sugar and it is now used in the food industry in some areas of the world, as a sugar substitute.
It is a native of South America, where it has been used as a sweetener for centuries, and does need protection from frost. The simplest option is to grow it in a pot and bring the pot into a conservatory, window sill or greenhouse over winter.
There are apparently three ways the plant is used.
1. The leaves are dried and crushed and used as a mild sweetener
2. Steeped in warm water overnight and the resulting syrup is used
3. Simmered in alcohol until the alcohol is removed, resulting in a very sweet syrup
Stevia is also a nice plant to grow as it has lime green leaves and pretty white flowers.
Safety Note: We are not recommending you eat this plant. Please do your own research before consuming any part of any plant!