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1
Preheat oven to 400F.
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Make cookies: Line two rimmed baking sheets with nonstick baking mats (such as Silpats), or with parchment.
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Sift together flour, cocoa powder, baking soda, baking powder, and salt.
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With an electric mixer, beat butter and sugar on medium speed until light and fluffy.
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Add eggs, buttermilk, and vanilla.
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Beat until well combined.
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On low speed, slowly add dry ingredients; mix until combined.
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Drop 1 1/2 tablespoons of batter onto prepared baking sheets, spacing 2 inches apart.
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Bake until set, about 8 minutes, rotating halfway through.
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(When the top of a cookie is gently touched, it should feel soft but not wet.)
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Cool completely on a wire rack.
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Make filling: With an electric mixer, beat peanut butter and butter on high speed until smooth.
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Reduce speed to low.
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Add confectioners sugar; mix until combined.
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Raise speed to high, and mix until fluffy and smooth, about 3 minutes.
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Season with salt, if desired.
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Assemble cookies: Spread 1 heaping tablespoon filling on the flat side of 1 cookie.
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Sandwich with another cookie.
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Repeat with remaining cookies and filling.
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Cookies can be refrigerated in single layers in airtight containers up to 3 days.
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Bring to room temperature before serving.
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In a small, heavy saucepan over medium heat, combine 1 cup sugar, the corn syrup, and the water; bring to a boil, washing down sides of the pan with a pastry brush dipped in water.
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Raise heat to medium-high.
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Cook without stirring until a candy thermometer reaches 230F.
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Meanwhile, with an electric mixer, beat egg whites on medium speed until soft peaks form.
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Gradually add remaining tablespoon sugar, and beat until peaks are very fluffy but still soft.
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When the syrup reaches 230F, reduce mixer speed to medium-low; pour syrup in a steady stream down the sides of the bowl.
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Increase speed to medium-high, and beat until cool, about 7 minutes.
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Add vanilla, and beat until fully incorporated.
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The frosting should be shiny and fluffy and hold stiff peaks.
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Use immediately.