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1
adjust oven rack to lower-middle position and heat oven to 375 degrees
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2
grease 2 quart souffle dish with 1 tablespoon softened butter, then coat dish evenly with 1 tablespoon sugar; refrigerate until ready to use
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3
melt chocolate and remaining 4 tablespoons butter in medium heatproof bowl set over saucepan of barely simmering water, stirring occasionally until smooth
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4
stir in liqueur, vanilla, and salt; set aside
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5
using stand mixer fitted with paddle, beat egg yolks and remaining 1/3 cup sugar on medium speed until thick and pale yellow, about 3 minutes
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6
fold into chocolate mixture
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7
using dry, clean bowl and whisk attachment, whip egg whites and cream of tartar on medium-low speed until foamy , about 1 minute.
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8
increase speed to medium-high and whip until stiff peaks form, 3 to 4 minutes
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9
using rubber spatula, vigorously stir one-quarter of whipped whites into chocolate mixture
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10
gently fold in remaining whites until just incorporated
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11
gently pour mixture into prepared dish, wiping any batter from rim with wet paper towel
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12
trace circle in batter with your finger, about 1/2 inch from edge of dish (this will help souffle rise nicely)
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13
bake until fragrant, fully risen, and exterior is set but interior is still a bit loose and creamy, about 25 minutes (use 2 large spoons to pull open top and peek inside)
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14
Serve immediately
