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1
To make cake: Preheat oven to 350u00b0. Butter an 18- x 12-inch jelly roll pan and line the bottom with parchment paper. Butter the parchment and dust the bottom and sides with flour, tapping out the excess.
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2
In a small pot, melt butter and bittersweet chocolate over medium-low heat, stirring often, until smooth, 4 to 5 minutes. Stir in espresso powder and set aside to let cool to room temperature. In a medium bowl, whisk together flour and salt. In a large bowl, beat sugar and eggs with an electric mixer on medium speed until thick and pale yellow, 3 to 4 minutes. Add vanilla and beat again until combined. Reduce speed to low and gradually add flour mixture, beating just until combined. Add melted-chocolate mixture in a steady stream, beating until thoroughly blended, and then stir in nuts, if using. Transfer to prepared pan, smooth out evenly, and bake until a toothpick inserted near the edge comes out clean, about 25 minutes. Set aside to let cool completely.
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3
Meanwhile, make ganache: Place semisweet chocolate in a large, heatproof bowl. In a small pot, heat heavy cream until steaming, pour over chocolate, and stir gently until smooth. Stir in liqueur, if using. Set aside to let cool to room temperature; alternatively, chill uncovered, stirring occasionally, until thickened but not set, 1 to 2 hours.
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4
Invert cake onto a clean surface, peel off and discard parchment paper, and halve crosswise to make 2 (9- x 12-inch) pieces. Invert 1 piece onto a wire rack set over a sheet tray. Spread with a generous layer of ganache, then top with the remaining cake, again inverted so that it sits right side up. Use remaining ganache to cover the top and sides evenly and chill, uncovered, until set, at least 2 hours and up to 2 days. Cut into bite-size squares and keep chilled until ready to serve.
