In a large mixing bowl whisk together melted butter and brown sugar. Add eggs and whisk vigorously for 1 minute. In a separate bowl mix together white chocolate, flour and baking powder. Add flour mixture to egg mixture and stir to blend. Stir in stout, stirring just until blended. Pour into prepared pan, spreading evenly, and bake for about 20 minutes or until done. Set on wire rack to cool.