Preheat oven to 350 F.
Butter and line a 9-inch square baking pan with parchment.
Over a pot of simmering water, melt butter halfway. Add white chocolate and stir until melted. Allow to cool to room temperature.
In a mixer fitted with the whisk attachment (or with an electric mixer), whip eggs, sugar and vanilla until pale and thick. Reduce speed to medium and add chocolate mixture.
In a separate bowl, sift together flour, baking powder and salt and stir into egg mixture, by hand. Stir in dried cranberries and walnuts and spoon into prepared pan.
Bake 45 minutes.
Allow to cool completely before cutting and dust with icing sugar. Brownies will keep for 4-6 days in an airtight container.