Preheat the oven to 340°F (170°C). Line two trays with baking paper.
Beat the butter and sugars until light and creamy, then add the condensed milk and beat until pale and smooth.
Gently beat in the vanilla, rolled oats, flour and baking powder, then add the white chocolate and cranberries and mix until combined.
Roll mixture into balls and place on the prepared trays. Press to flatten gently with a wet fork.
Bake for 25-30 minutes until golden brown. Cool for 5 minutes on the trays, then transfer to a wire rack to cool completely. These will keep in an airtight container for up to 10 days.