Grasshopper Squares

Preparation

1. Make brownie layer:

   Step 1

   Put oven rack in middle position and preheat oven to 375°F. Lightly butter a 13- by 9-inch baking pan and line with 2 crisscrossed sheets of foil, leaving a 2 inch overhang on all sides. Butter foil.

   Step 2

   Melt butter and chocolate with brown sugar in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until smooth. Remove from heat. Whisk in eggs and vanilla until combined. Whisk in flour, cocoa, and salt until just combined.

   Step 3

   Spread batter evenly in baking pan and bake until set and a wooden pick inserted in center comes out with moist crumbs adhering, about 20 minutes. Cool completely in pan on a rack, about 1 1/2 hours.

2. Make mint ganache:

   Step 4

   Bring cream to a simmer in a 2- to 3-quart saucepan and remove from heat. Pour over white chocolate in a bowl. Let stand 1 minute, then whisk until smooth. Stir in crème de menthe and extract and chill, covered, stirring occasionally, until thick, about 1 hour.

3. Make chocolate ganache:

   Step 5

   Bring cream to a simmer in a 2- to 3-quart saucepan and remove from heat. Pour over bittersweet chocolate in a bowl. Let stand 1 minute, then whisk until smooth. Chill, covered, stirring occasionally, until thick, about 30 minutes.

4. Assemble layers:

   Step 6

   Spread mint ganache over top of cooled brownie in a thin even layer using offset spatula, then chill until firm but still slightly sticky, about 30 minutes.

   Step 7

   Spread chocolate ganache over mint and chill until firm, about 2 hours.

   Step 8

   Lift dessert out of pan using foil overhang. Run a heavy knife under hot water and wipe dry, then trim edges of dessert (1/4 inch off each side). Cut dessert into squares and peel from foil.