Line an 8-inch square pan with foil, extending foil over edges of pan; set
pan aside. Heat butter in a medium saucepan over medium heat until the butter
turns the color of light brown sugar. Stir frequently. Remove from heat; cool
slightly. Set aside 2 tablespoons of this butter for the icing. Stir vanilla
extract into remaining butter; chill mixture for 1 hour or until firm.
Meanwhile, combine flour and brown sugar in medium mixing bowl. Cut firm
butter mixture into flour with a pastry blender until it resembles fine crumbs
and the mixture starts to cling. Press dough firmly into bottom of prepared
pan. Bake at 325 degrees F, about 25 minutes, or until firm but not brown. Immediately
use foil to lift shortbread from pan. Cut into bars. Use foil to transfer bars
to a wire rack. Cool bars, still on foil and now on the rack, completely.
For icing, combine reserved butter, confectioners' sugar and enough
milk to make an icing of desired consistency. Drizzle icing over cooled shortbread.
Sprinkle with cashews.