1. In a medium mixing bowl, beat together the butter and sugar until fluffy.
2. Beat in the egg, milk, and vanilla until combined.
3. Add the flour, baking powder, and salt, and stir until just combined.
4. Fold in the cranberries and white chocolate chips.
5. Scoop batter into greased or paper lined muffin tins, filling each opening about 3/4 full.
6. Bake at 350 for 20-22 minutes, until a toothpick inserted in the center of the muffin comes out clean.