By Rosie Hawthorne on December 23, 2019
Cream butter and brown sugar in a large bowl with 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 well combined. Stir in flour and salt.
Spread batter evenly in an ungreased 10 ½- by 15 ½-inch baking pan and bake in middle of oven until top puffs slightly and sides pull away from edge of pan, about 18-20 minutes.
While base is baking, melt chocolate in a double boiler, stirring occasionally, then remove from heat.
Spread chocolate over warm base and immediately sprinkle with cashews. Cool in pan on rack. Chill until chocolate is firm, then cut into bars.