You might be tempted to bake 2 sheets of bars at a time in the upper and lower thirds of the oven. Don't give in to this urge — they must go in the middle of the oven to cook through and brown evenly.
 

Active time: 20 min Start to finish: 1 1/2 hr

2 sticks (1 cup) cannabutter, softened
1 cup packed brown sugar
1 large egg yolk
1 1/2 teaspoons vanilla
3 tablespoons instant-espresso powder* dissolved in 2 tablespoons boiling water
2 cups all-purpose flour
1/2 teaspoon salt
8 oz fine-quality bittersweet chocolate (not unsweetened), finely chopped
3/4 cup salted roasted cashews (4 oz), chopped

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.

Cooks' note:
• Cookies keep, layered between sheets of wax or parchment paper and chilled in an airtight container, 2 weeks.

* Available at some supermarkets or specialty stores. If you can't find espresso powder any kind of instant coffee may be substituted.

Makes about 4 dozen bars.