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Preheat your oven to 350.
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Line a 9x13 baking dish with parchment, and spray the parchment and pan with baking spray.
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Melt the butter and 12oz chocolate chips in a double boiler. Once everything is melted, stir to incorporate, and then set aside to cool.
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In a large bowl, combine the eggs, sugar, espresso powder, and vanilla. Beat with a whisk until combined.
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Mix in the cooled chocolate/butter mixture.
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Sift in the flour, cocoa powder, salt, and baking powder. Fold to combine making sure not to over mix.
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Stir the remaining cup of chocolate chips and the walnuts into the batter.
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Pour the brownie batter into your prepared pan, and bake at 350 for 30-40 minutes.
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Allow to cool completely before slicing.