Fill a saucepan with about 1 inch of water and bring it to a gentle simmer over medium-low heat. Thoroughly wash and fully dry a stainless-steel mixing bowl (no grease) and have your stand mixer and attachments ready. Ensure the 1 1/2 cups (3 sticks) unsalted butter is softened.

In the clean, dry mixing bowl whisk together 7 oz egg whites (from 7 large eggs) and 2 cups granulated sugar until combined.

Set the mixing bowl over the simmering saucepan so the bowl sits over the steam but does not touch the water (double boiler). Whisk constantly until the mixture reaches 160°F and the sugar is fully dissolved (about 3 minutes). To check dissolution, rub a small amount between your fingertips — it should feel smooth, not gritty. The mixture will be hot.

Remove the bowl from the saucepan. Wipe any water or condensation from the bottom of the bowl with a clean towel, then place the bowl on the stand mixer fitted with the whisk attachment.

Beat on medium-high speed until the meringue forms stiff, glossy peaks and the outside/bottom of the bowl feels completely at room temperature (about 15–20 minutes). It is important the meringue is cool; warm meringue will melt the butter.

Switch to the paddle attachment and reduce the mixer to medium speed. Add the softened butter 1 tablespoon at a time, adding each tablespoon just as it is absorbed by the meringue.

After all the butter is added, scrape down the bowl and beat on medium-high for about 3 minutes, until the buttercream is thick and whipped. If it looks lumpy, thin, or separated, continue beating until smooth, thick, and fully combined.

Add 2 teaspoons vanilla extract and 1/4 teaspoon fine sea salt and mix on medium-high until fully incorporated (about 1 minute).