Mocha Cashew Bars

1 cup ( 2 sticks ) unsalted butter , softened
1 cup brown sugar , packed
2 cups all purpose flour
1 large egg yolk
2 teaspoons vanilla
4-5 tablespoons instant coffee / espresso dissolved in boiling water
1/2 teaspoon salt
10oz high quality bittersweet chocolate (ie. Lindt) finely chopped
1 cup salted roasted cashews , chopped

1. Preheat oven to 350 degrees F.
2. Beat together butter and brown sugar in a large bowl with an electric mixer at moderately high speed until pale and fluffy, 3-4minutes. Beat in yolk and vanilla, then slowly add the coffee mixture , beating until combined well. Add the flour and salt and mix at low speed until just combined.
3.Spread the batter evenly in an ungreased 15 1/2 by 10 1/2 - inch baking pan 1 inch deep. Bake in middle of oven until the top puffs slightly and the sides pull away from edge of pan , 15-22 minutes. Batter may burn easily , please watch carefully.
4.While bar base is baking , melt chocolate in a double boiler or a metal bowl set atop a sauce pan of simmering water, stirring occasionaly, then remove from double boiler or bowl from heat.
Spread chocolate over the warm base and immediately top with cashews. Cool completely in pan on a rack , then cut into small bars should yield 48 bars. Chill until chocolate is firm , 15 - 20 minutes.