5 large egg whites
1 cup plus 2 tablespoons granulated sugar
1 pound (4 sticks) unsalted butter, softened, cut into cubes
1 1/2 teaspoons pure vanilla extract
1 tablespoon vanilla bean paste OR 1 vanilla bean, split and scraped
pinch of salt
Wipe the bowl of an electric mixer with vinegar on paper towel, to remove any trace of grease. Add egg whites and sugar, and simmer over a pot of water (not boiling), whisking constantly, until temperature reaches 140 degrees F, or if you don't have a candy thermometer, until the sugar has completely dissolved and the egg whites are hot.
With whisk attachment of mixer, begin to whip until the mixture is thick, glossy, and cool. Switch over to paddle attachment and, while mixing on medium speed continously, add softened butter in chunks until incorporated, and mix until it has reached a silky smooth texture (if curdles, keep mixing and it will come back to smooth). Add vanilla and salt, mix well.
Keep in airtight container in refrigerator for up to one week, leaving out at room temperature when needed, rewhipping in mixer for 5 minutes.
Can freeze for up to 6-8 weeks.