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1
Prebake store-bought crust according to package directions; let it cool; refrigerate until ready to use.
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2
Over very low heat, combine 1/2 cup of the coffee and 2 tablespoons butter in a medium saucepan, preferably nonstick.
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3
As the butter starts to melt, add the chocolate; let the mixture stand on the heat for 3-4 minutes, stirring a few times.
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4
When the chocolate has melted, remove from the heat, whisking to smooth.
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5
Whisk the remaining 2 tablespoons butter into the mixture, adding about 1/2 tablespoon at a time.
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6
Transfer the pan to a wire rack and let cool, whisking occasionally.
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7
While the mixture is still warm, whisk in the egg yolks.
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8
Put the saucepan back over very low heat and heat the mixture, whisking virtually nonstop, for 3-4 minutes to cook the yolks.
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9
Remove pan from heat and stir in the vanilla; let cool to room temperature, then refrigerate, whisking occasionally.
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10
Pour the remaining 1/4 cup coffee into a small saucepan and sprinkle the gelatin over it; set aside for 5 minutes to soften.
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11
Gently heat the mixture, whisking, until the gelatin is dissolved, 1-2 minutes.
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12
Whisk this into the chocolate mixture and return the mixture to the refrigerator, whisking occasionally, 30-60 minutes, until it has the consistency of a very thick fudge sauce.
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13
Using an electric mixer, beat the egg whites until they hold soft peaks; beat in the cream of tartar and salt.
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14
Gradually add the sugar, beating on high speed until the whites are thick, glossy, and firm but not dry.
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15
Transfer the chocolate mixture to a large bowl and fold in 1/3 of the egg whites.
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16
Add the rest of the egg whites and continue to fold, until no white streaks remain.
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17
Wash and dry beaters; Using a chilled bowl and chilled beaters, beat the heavy cream with the mixer until stiff but not grainy.
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18
Fold the whipped cream into the chocolate mixture until evenly combined.
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19
Scrape filling into the chilled pie shell, smoothing the top with a spoon.
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20
Cover loosely with tented foil and refrigerate for at least 4 hours, preferably 6.
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21
Serve garnished whipped cream, if desired.
