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1
Combine the eggs and brown sugar in the metal bowl of a standing mixer. (Alternatively, use a metal mixing bowl and a handheld electric mixer, but be prepared for a workout.)
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2
Fill a large saute pan or frying pan with water and bring to a simmer over medium-high heat.
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3
Place the mixing bowl in the simmering water and whisk the eggs and sugar constantly until the sugar is completely dissolved and the mixture is thick and fluffy and very hot, 3 to 4 minutes.
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4
Use a candy thermometer to check the temperature; it should be anywhere between 120u00b0F and 140u00b0F (49u00b0C to 60u00b0C).
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5
Remove the bowl from the simmering water and, using the whisk attachment, beat the eggs at medium-high speed until they are tripled in volume and form soft peaks and the bottom of the bowl is completely cool to the touch, about 10 minutes.
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6
Beat in the vanilla and salt.
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7
While the eggs are mixing, unwrap the individual sticks of butter and rewrap them loosely in plastic wrap.
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8
Pound the butter 5 or 6 times with a rolling pin (this is my favorite part!), or until the butter is soft and malleable but still cool.
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9
With the mixer speed still on medium-high, add the butter, approximately 2 tablespoons at a time, to the egg mixture, beating in each addition until it is incorporated.
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10
When all the butter has been incorporated, slowly dribble in the bourbon. Don't start to panic if the buttercream seems too liquid or looks curdled as you beat in the butter. It will magically emulsify into a smooth, creamy frosting by the time the last little bit of butter is mixed inches.
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11
Have faith; it's worth it.
