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Preheat oven to 325 F.
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Line two or three baking sheets with parchment paper (a must).
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Melt butter and chocolate in a double boiler or a metal bowl set over a pan of simmering water.
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Remove from heat and whisk in dissolvd espresso.
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Let this mixture cool for several minutes, then whisk in egg yolks.
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Beat egg whites with salt in a medium bowl on low speed until they are frothy.
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Increase speed to high and slowly add brown sugar, beating until whites form soft peaks.
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Stir in one-third of the beaten egg whites into chocolate mixture to lighten it, then gently fold in the rest of the whites.
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The batter should be light and fluffy- a mousse.
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Drop batter by the tablespoon onto parchment paper, spacing them 2 1/2 inches apart.
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Bake the cookies for 5 to 8 minutes, until they are puffed and not too glossy.
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Slide parchment paper (with cookies still on it) onto cooling racks.
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Once the cookies have cooled, leave them on the paper and freeze them until they're firm enough to remove.
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Keep them frozen until serving.