Grease a 9-inch square pan. In a saucepan melt the butter and add the brown sugar. Cook over lowheat for 2 minutes, stirring constantly. Add the milk and continue cooking, stirring until the mixture boils. Remove from the heat and allow to cool. Gradually add the confectioners' sugar until the mixture is of fudge consistency. Add the cashews and stir well. Spread in the prepared pan and cool.