Position the rack in lowest third of oven and preheat to 180ºC . Butter sides of 23 cm square baking pan with 5 cm-high sides.
Fold 46 cm x 30 cm piece in half crosswise. Line pan with foil, allowing foil to extend over two sides. Butter foil. Melt both chocolates and butter in heavy medium saucepan over very low heat, stirring until smooth. Cool. Sift flour and salt into small bowl. Mix cream, espresso powder and creme de menthe in small cup. Using electric mixer, beat eggs and sugar in large bowl until frothy. Fold in coffee mixture and then melted chocolate. Fold in dry ingredients. Spread batter evenly in prepared pan. Bake until toothpick inserted in center comes out with some moist crumbs still attached, about 25 minutes (surface may crack). Gently press down on any raised surfaces to flatten slightly. Cool 30 minutes on rack.
FOR GLAZE: Melt both chocolates and butter in heavy small saucepan over low heat, stirring until smooth. Let stand until cool enough to spread. Spread glaze over brownies. Let stand 4 hours at room temperature. Lift brownies from pan using foil sides. Cut into squares. (Can be prepared 1 day ahead. Store in airtight container.)