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Slice each chicken breast horizontally to butterfly it. Cut through the center to create four separate pieces. If needed, gently pound the thicker areas until each piece is approximately ½ to ¾ inch thick.
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Pat the chicken dry with paper towels. Season both sides with salt.
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Heat a large skillet over medium-high heat. Add the olive oil and 1 tablespoon of butter.
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Once the oil is shimmering, add the chicken in a single layer. Cook in batches if necessary to avoid overcrowding the skillet.
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Cook the chicken undisturbed for 3–4 minutes per side, or until golden brown and the internal temperature reaches approximately 160°F.
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Transfer the chicken to a clean plate and loosely cover it with foil. It will continue cooking as it rests.
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Reduce the heat to medium. Add the remaining tablespoon of butter and the minced garlic. Cook for approximately 30 seconds, stirring constantly.
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Remove the skillet from the burner and carefully add the brandy or cognac. Return the skillet to the heat.
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Scrape the browned bits from the bottom of the skillet. Simmer the brandy for 1–2 minutes, or until reduced by approximately half.
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Reduce the heat to medium-low. Stir in the heavy cream, Dijon mustard, ground peppercorns, and rosemary.
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Simmer the sauce gently for 3–5 minutes, stirring occasionally, until it becomes smooth, glossy, and thick enough to coat the back of a spoon.
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Taste the sauce and adjust the salt, mustard, and peppercorns as needed.
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Return the chicken and any accumulated juices to the skillet. Spoon the sauce over the chicken and simmer for 1–2 minutes, or until the chicken is warmed through and reaches 165°F.
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Remove the skillet from the heat. Garnish with finely chopped chives and serve immediately.