Because it does take some time to make and freezes well, I make a larger batch. You can divide this recipe in 2 or even 3.
Yield: 15 cups (enough to frost and fill approximately three 8" round cakes)
This triple batch fits perfectly in my 4.5 QT KitchenAid Mixer.
16 large egg whites (30g each--total 450g, or 2 cups)
4 cups granulated sugar (800g)
5 cups (2.5 lbs, 10 sticks) of unsalted butter, softened but cool, cut into cubes
30 ml (2 tablespoons) pure vanilla extract
1/4 tsp salt
Wipe the bowl of an electric mixer, whisk attachment, small bowl, and whisk with paper towel and lemon juice (or vinegar), to remove any trace of grease. Add egg whites and sugar, and simmer over a pot of water (not boiling), whisking constantly but gently, until temperature reaches 160 degrees F, or if you don't have a candy thermometer, until the sugar has completely dissolved and the egg whites are hot (you can feel a drop in between your fingers to ensure no granules.).
Take off of stove, and place bowl back on electric mixer fitted with whisk attachment, begin to whip until the mixture is thick, glossy, and neutral (you can feel outside of the bowl to test temperature). Switch over to paddle attachment and, while mixing on low speed continously, add butter one cube at a time 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. You can also add a wide variety of flavourings, extracts, and more. If buttercream is too runny, the butter was possibly too soft--place into the refrigerator for about 15 minutes, then beat again. If still too runny, add a few more cubes of butter and keep beating. Use immediately, or refrigerate/freeze.
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. To thaw, place on counter overnight, and rewhip for 5 minutes with paddle attachment in an electric mixer.
If buttercream still doesn't have its satiny finish after rewhipping, microwave 1/3 of the buttercream for approximately 10 seconds and add to remaining buttercream in mixer bowl, beating for a few moments to incorporate.
The variations are almost endless, but here are some delicious options:
Chocolate Buttercream: Add 300 g (1 1/2 cups before melting) melted bittersweet high quality chocolate (the best you can get--I use Callebaut) for every 5 cups of vanilla Swiss Meringue Buttercream and beat until incorporated.
Strawberry Buttercream (or any other berry version): Add fruit puree to taste (approximately 1/2 cup for 5 cups of SMB).
Vanilla Bean Buttercream: Add 1 tablespoon of Vanilla Bean Paste OR 1 vanilla bean, scraped for every 5 cups of SMB, and beat until incorporated.
You can also add liqueurs, extracts, and other flavourings, as well as any food gel colours to achieve any desired colour. Just a note that SMB has a yellow tone, due to the butter, so it really can't be pure white.