Instructions

Adjust oven rack to middle position; heat oven to 325 degrees. Cut 18-inch length foil and fold lengthwise to 8-inch width. Fit foil into length of 13- by 9-inch baking dish, pushing it into corners and up sides of pan; allow excess to overhang pan edges. Cut 14-inch length foil and, if using extra-wide foil, fold lengthwise to 12-inch width; fit into width of baking pan in same manner, perpendicular to first sheet. Spray foil-lined pan with nonstick cooking spray.
Spread nuts (if using) evenly on rimmed baking sheet and toast in oven until fragrant, 5 to 8 minutes. Set aside to cool.
Whisk to combine flour, salt, and baking powder in medium bowl; set aside.
Melt chocolate and butter in large heatproof bowl set over saucepan of almost--simmering water, stirring occasionally, until smooth. (Alternatively, in microwave, heat butter and chocolate in large microwave-safe bowl on high for 45 seconds, then stir and heat for 30 seconds more. Stir again, and, if necessary, repeat in 15-second increments; do not let chocolate burn.) When chocolate mixture is completely smooth, remove bowl from saucepan and gradually whisk in sugar. Add eggs one at time, whisking after each addition until thoroughly combined. Whisk in vanilla. Add flour mixture in three additions, folding with rubber spatula until batter is completely smooth and homogeneous.
Transfer batter to prepared pan; using spatula, spread batter into corners of pan and smooth surface. Sprinkle toasted nuts (if using) evenly over batter and bake until toothpick or wooden skewer inserted into center of brownies comes out with few moist crumbs attached, 30 to 35 minutes. Cool on wire rack to room temperature, about 2 hours.
When the brownies are cool, leave them in the pan.

for mint frosting

In the bowl of a standing mixer or with a handheld mixer, beat butter and confectioners' sugar at low speed until just incorporated, then increase the speed to medium and beat until smooth and fluffy, about 1 1/2 minutes. Add 1 tablespoon milk and mint extract and continue to beat until combined, about 30 seconds, adding up to 1 additional tablespoon milk if necessary to achieve a soft spreadable consistency. Using an offset spatula, spread the mint frosting evenly onto the cooled brownies, cover with foil, and refrigerate until firm, about 1 hour

for chocolate glaze

Melt chocolate and butter in a medium heatproof bowl set over a saucepan of barely simmering water, stirring occasionally, until smooth; set aside to cool slightly, about 10 minutes.
Pour the chocolate glaze on the frosted brownies; using an offset spatula, spread the glaze into an even layer. Cover with foil and refrigerate until firm, about 1 hour. Remove the brownies from the pan by lifting the foil overhang, cut into 2-inch squares, and serve. (Store leftovers in an airtight container in the refrigerator.)