Preheat oven to 350F.

In a large bowl, beat together softened butter and packed brown sugar with an electric mixer at moderately high speed until pale and fluffy, about 3 minutes.

Beat in egg yolk and vanilla extract until combined.

Gradually add the dissolved instant espresso mixture, beating until well combined.

Add flour and salt to the mixture.

Mix at low speed until just combined, being careful not to overmix.

Spread the batter evenly in an ungreased 15 1/2- by 10 1/2-inch baking pan (1 inch deep).

Bake in the middle of the oven until the top puffs slightly and the sides pull away from the edge of the pan, 16 to 22 minutes. Watch carefully toward the end of baking to prevent burning.

While the base is baking, melt the bittersweet chocolate in a double boiler or a metal bowl set over a saucepan of barely simmering water, stirring occasionally.

Once melted, remove the top of the double boiler or bowl from the heat.

Spread the melted chocolate evenly over the warm baked base.

Immediately sprinkle the chopped salted roasted cashews over the melted chocolate.

Cool completely in the pan on a wire rack.

Cut into 48 bars.

Chill the bars until the chocolate is firm, about 15 minutes.