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Preheat oven to 350°F.
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Line 2 large baking sheets with parchment paper.
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Put powdered sugar, almond flour, and salt in a food processor pulsing several times. Continue for about 30 seconds or pulse until the ingredients are fine and well combined.
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Using a flour sifter, sift the combined powdered sugar/almond flour/salt mixture into a large bowl. This may take awhile.
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Place 3 egg whites in a clean bowl of a stand mixer fitted with a whisk attachment. Beat on medium speed until opaque and foamy, about 30 seconds.
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Now add a pinch of cream of tartar, increase the speed to medium-high, and continue to beat until the egg whites are white in color and hold the line of the whisk, about 1 minute.
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Continue to beat—slowly adding the granulated sugar and vanilla extract until the sugar is combined, the peaks are stiff, and the whites are shiny, about 1 minute more. Do not over whip. Transfer the meringue to a large bowl.
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Take a rubber spatula, and now gently fold the dry mixture into the egg whites in four different scoops until the dry ingredients are just combined.
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Spoon batter into pastry bag fitted with ½-inch plain round tip. Pipe batter onto each prepared sheet in 12 walnut-size mounds, spacing mounds apart because cookies will spread slightly.
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Bake cookies, 1 sheet at a time, until firm to touch in the center and dry and cracked on top, about 7 minutes .... pull the pan out and then rotate and bake for another 7 minutes.
IMPORTANT: Slide parchment with cookies onto work surface; cool cookies completely. Repeat with remaining batter, cooling sheets fully and lining with clean parchment for each batch.