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1
Preheat the oven to 350F, with a rack in the middle.
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2
Put the butter and brown sugar in the bowl of an electric mixer fitted with the paddle attachment.
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3
Mix on medium speed until fluffy, 2 to 3 minutes.
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4
Mix in the egg whites, one at a time.
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5
Mix in the flour, salt, and vanilla.
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6
Line a baking sheet with a Silpat baking mat or parchment paper.
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7
Spoon 2 tablespoons batter onto the baking mat.
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8
Using an offset spatula, spread the batter into a 7-inch circle, with the edges slightly thicker than the center.
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9
Repeat, making a second circle on the mat.
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10
Bake, rotating the sheet halfway through, until the edges of the cookies turn golden, about 9 minutes.
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11
Using a small spatula, immediately transfer 1 cookie to a small bowl (about 5 1/2 inches in diameter and 3 inches deep).
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12
Gently mold the warm cookie to the shape of the bowl, pressing the bottom down to flatten.
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13
Let stand in the bowl 30 seconds; remove.
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14
Repeat with the remaining cookie.
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15
If the cookies become too cool to shape, return them to oven for 20 seconds.
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16
Repeat, baking the remaining batter and forming it into bowls.
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17
If using just 1 baking sheet, let it cool before spreading the next batch of batter onto it.
