2sticks(226g; 8 ounces) unsalted butter, softened at room temperature
1teaspoonvanilla extract
zest of 1 lemon
pinch of salt (1/4 teaspoon kosher salt)
Instructions
Make a double boiler (a small saucepot filled with 1/3 of water, heated to a simmer with a small heatproof bowl resting on top- it should not be touching the water).
Add in egg whites and sugar into bowl and heat until sugar is completely dissolved and not grainy. The mixture should be quite warm (160F, but you don’t have to use a thermometer here)
Pour egg white/sugar mixture into bowl of standing mixer. Using whisk attachment, mix on high speed (speed 8) until a meringue with soft peaks (looks like marshmellow fluff) has formed and the outside of the bowl is room temperature to touch (about 70F), about 5-10 minutes. If it’s too hot, the butter will melt when you add!
Add in softened butter, a bit at a time, with mixer running at low to medium speed. Mix until smooth and incorporated.
Add in lemon zest, a pinch of salt, and vanilla. Mix until smooth. Use immediately or store in fridge but bring to room temp when using.