Combine butter and sugars and mix for about 5 minutes, until light and fluffy. Add egg and vanilla. In a separate bowl, combine flour, baking soda and salt. Add dry ingredients to the wet ingredients until just combined. Stir in white chocolate, cranberries and blueberries. If time permits, allow cookie dough to sit covered in the fridge for a few hours. Scoop onto parchment lined or greased cookie sheet and sprinkle very, very lightly with kosher salt.
Bake at 375 degrees for 8 to 10 minutes, until edges are lightly browned.