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1
Put oven racks in upper and lower thirds of oven and preheat oven to 300u00b0F. Line 2 large baking sheets with parchment paper.
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2
Finely grind almonds with confectioners sugar in a food processor.
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3
Beat whites with salt in mixer at medium-high speed until they just hold soft peaks.
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4
Reduce speed to medium, then add granulated sugar, a little at a time, beating, and continue to beat until whites just hold stiff, glossy peaks. Fold almond mixture in 2 batches into whites until just combined, then fold in vanilla and almond extracts. Transfer meringue to pastry bag and pipe 16 (1 1/2-inch-wide) mounds 1 inch apart on each lined baking sheet (32 total). Smooth tops of mounds with a wet fingertip.
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5
Bake, switching position of sheets halfway through baking, until macaroons are puffed and tops appear dry, 15 to 17 minutes. (Macaroons should be crisp on the outside and chewy on the inside.) Slide parchment with macaroons onto racks and cool 10 minutes, then peel macaroons from parchment, transferring to a rack to cool completely, about 15 minutes.
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6
Vigorously whisk together whites, sugar, and salt in cleaned mixer bowl set over a 4-quart pot of barely simmering water until sugar is dissolved and mixture is warm. Put bowl in mixer and beat with cleaned whisk attachment at medium-high speed until whites hold stiff, glossy peaks. Continue to beat, scraping down side of bowl with a rubber spatula, until meringue is completely cool to the touch, about 5 minutes.
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7
With mixer at medium speed, gradually add butter 1 piece at a time, beating well after each addition until incorporated. (If buttercream looks soupy after some butter is added, meringue is still too warm. If so, briefly chill bottom of bowl in a large bowl filled with ice water for a few seconds before beating in remaining butter.) Continue beating until buttercream is smooth. (Mixture may look curdled before all of butter is added but will come back together by the time beating is finished.) Add vanilla and almond extracts and beat 1 minute more.
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8
Transfer buttercream to cleaned pastry bag and pipe about 1 tablespoon onto flat sides of 16 macaroons, then top with remaining 16 macaroons.
