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1
Preheat oven to 350u00b0 F. Butter a 13- by 18-inch half-sheet pan.
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2
Using an electric mixer on medium-low speed, mix the brown sugar and butter in a large bowl until well blended but not too fluffy.
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3
Add the eggs one at a time, mixing well after each addition and scraping down the sides of the bowl occasionally. Mix in the vanilla and salt.
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4
In a medium bowl, stir together the flour, cocoa powder, espresso powder, and nuts. Pour into the sugar mixture and mix on low speed.
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5
Pour the batter into the prepared pan and add the chocolate chips.
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6
Bake for 23 1/2 minutes (assuming your oven is calibrated properly). The batter will still be soft in the middle of the pan. Allow the brownies to cool thoroughly before frosting.
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7
Combine all the ingredients except the chocolate chips in a heavy saucepan over medium heat. Cook, stirring constantly, until the mixture comes to a boil, then for 1 additional minute. Remove from heat, add the chocolate chips, and stir until smooth. Pour the frosting evenly over the cooled brownies. When the frosting has set, cut brownies into squares of the desired size. Store the brownies in the refrigerator, separating the layers with parchment paper, for up to 3 days.
