Swiss Meringue Buttercream

• Prep35 min
• Cook10 min
• Total45 min
• Serves24

Ingredients

• 130 g egg whites (from about 4 large eggs)
• 250 g sugar (1 1/4 cups)
• 1 pinch salt
• 400 g unsalted butter (1 3/4 cups, 3 1/2 sticks), softened but cool, cut in 2.5 cm (1 in) cubes
• 2 tsp vanilla extract

Instructions

1. Cut the butter into cubes and leave it out 30 to 45 minutes, until it bends without cracking and a finger leaves a dent, but it still feels cool, about 18-20°C (65-68°F). Wash and dry the mixing bowl and butterfly whisk very well and wipe both with a paper towel dampened with vinegar or lemon juice: any trace of fat or yolk keeps egg whites from whipping.
2. Insert the butterfly whisk, add the egg whites, sugar and salt and whip 10 min/70°C (160°F)/speed 3.5. Heating the whites to 70°C (160°F) makes them safe to eat without baking and dissolves the sugar completely. Rub a drop between your fingers: it should feel smooth, with no grains. If you still feel some, whip 2 min/70°C (160°F)/speed 3.5 more.
3. Remove the measuring cup and whip 12 min/speed 4 without heat, until the meringue is thick, glossy and holds stiff peaks, and the outside of the bowl feels only slightly warm, about body temperature or cooler. If it still feels warm, whip 3 min/speed 4 more: a warm meringue melts the butter.
4. Whip 5 min/speed 4, dropping the butter cubes one at a time through the lid opening, a few seconds apart. Adding it slowly gives the meringue time to take in each piece.
5. Partway through, the buttercream may look curdled, like cottage cheese. That is normal: the butter and the meringue have not come together yet. Keep whipping 3 min/speed 4 and it turns smooth and silky. If it is still curdled after that, it is too cold: whip 1 min/37°C (98°F)/speed 4, then 2 min/speed 4 without heat.
6. If it looks soupy, thin and runny instead, the meringue or the butter was too warm. Lift out the mixing bowl, cover it and refrigerate 15 to 20 minutes, until the edges feel firm, then put it back in the TM7 and whip 3 min/speed 4.
7. Add the vanilla and whip 1 min/speed 4, until smooth. For smooth cake sides, stir it by hand with the spatula for 1 minute to push out the big air bubbles, then use it right away to pipe or spread.

Tips

• Keep the buttercream in an airtight container up to 1 week in the fridge or 3 months in the freezer (thaw it overnight in the fridge). Frosted cupcakes keep 2 days covered in an air-conditioned room, or 4 days in the fridge; take them out 30 minutes before serving so the buttercream softens.
• To rewhip chilled buttercream, leave it at room temperature 2 to 3 hours, until soft all the way through, then whip 3-5 min/speed 4 with the butterfly whisk. It looks curdled at first, then turns silky again. If the center is still cold, use the 37°C (98°F) trick from step 5.
• Flavors: add 120 g (4 oz) melted dark chocolate, cooled until barely warm, 1 tsp almond or coconut extract, or the finely grated zest of 1 lemon or orange with the vanilla. For a fruity pink buttercream, mill 15 g (3/4 cup) freeze-dried strawberries or raspberries in the dry bowl 10 sec/speed 10 before step 1, set the powder aside and add it with the vanilla.
• This buttercream is ivory because of the butter. Gel colors work best: add them in the last minute of whipping, a little at a time, and wait a few minutes, because the color deepens as it sits. For a whiter look, add a tiny dot of violet gel on a toothpick; for deep reds or black, color it a day ahead.