Mocha Toffee Cashew Bars

Preheat oven to 350°F. Beat together butter and brown sugar in a large bowl with an electric mixer at moderately high speed until pale and fluffy, about 3 minutes. Beat in yolk and vanilla, then gradually add espresso mixture, beating until combined well. Add flour and salt and mix at low speed until just combined. Spread batter evenly in an ungreased 15 1/2- by 10 1/2-inch baking pan (1 inch deep) and bake in middle of oven until top puffs slightly and sides pull away from edge of pan, 16 to 22 minutes. (Watch carefully toward end of baking; base can burn easily.)While base is baking, melt chocolate in a double boiler or a metal bowl set over a saucepan of barely simmering water, stirring occasionally, then remove top of double boiler or bowl from heat. Spread chocolate over warm base and immediately sprinkle with cashews. Cool completely in pan on a rack, then cut into 48 bars. Chill until chocolate is firm, about 15 minutes.