1) Preheat the oven to 190C/Gas Mark 5.
2) Put the cranberries and orange juice in a small saucepan, and bring just to a simmer over medium heat. Remove berries from the heat, and set aside to cool and plump. Drain off any excess liquid and set aside.
3) Lightly brush a 12-muffin tin with butter. Sift the flour, baking powder, and salt into a medium bowl and set aside.
4) In a standing mixer fitted with the paddle attachment, or with an electric hand-held mixer in a large bowl, cream the butter, orange zest, and 135g sugar until light and fluffy, about 2 mins. Scrape down the sides of the bowl with a rubber spatula. Add the eggs, one at a time, beating well after each addition. Remove the bowl from the mixer.
5) Fold the flour in 3 parts into the butter mixture, alternating with the milk in 2 parts, until just combined. Fold in the cranberries. Do not over mix. Divide the batter evenly into the muffin tin and sprinkle the tops with sugar. Bake until golden brown, about 25 mins. Cool muffins in the pan on a rack. Serve warm.