I’m confused too as I have been preoccupied with a lot of turmoil around my house.
So re-read everything. I see two causes for the Swiss meringue buttercream failure: incorrect amount of ingredients and incorrect beating of egg whites.
Ingredients amounts:
Ingredients vary in weight. For example:
1 cup of sugar weighs 7 oz or 200 g.
1 cup of flour weighs 4.25oz or 120 g.
1 cup of powdered sugar weighs 4 oz or 113 g.
1 cup of butter weighs 8 oz or 226 g.
The basic ingredients used in Swiss meringue buttercream weigh the following:
1 large egg white weighs approx 1.07 oz or 30 g
1 stick butter weighs 4 oz or 113 g
1 cup sugar weighs 7 oz or 200 g.
The recipe you used came out to the following weights:
7 egg whites = approx. 7.5 oz or 210 g
2 cups sugar = 14 or 400 g
2 1/2 sticks butter = 10 oz or 254 g
The ratios for Swiss meringue buttercream BY WEIGHT, not volume:
1 part egg whites
2 parts sugar
3 parts butter
The ratios you used:
1 part egg whites
1.90 sugar
1.20 parts butter.
While the amount of sugar you used was less than 2 parts, it was still enough to work. You can use +/- 10% - 15% of an ingredient on most recipes without any problems. When you get move beyond +/- 10% - 15%, the recipe will destabilize.
And that’s what happened with your recipe. The butter was far beyond the +/-10% - 15%. For the amount of egg whites you used, you needed 5 1/2 sticks of butter or 630 g. You were short 3 sticks of butter.
Beating:
Beating is separated into three phases: meringue, butter addition, and emulsification.
Phase 1: Meringue 3-5 minutes depending on mixer speed and amount of egg whites. If you beat your egg whites for 10 minutes, they were over-beaten. Over beaten egg whites will collapse.
Phase 2: Butter addition approximately 5 minutes depending on temperature of butter and amount of butter.
Phase 3: Emulsification approximately 8 - 10 minutes. Emulsification is the suspension of two ingredients that do not normally mix well together. In this case, fat into water (egg whites and butter both contain lots of water).
Here’s a basic Swiss meringue buttercream. This makes enough for 12- 14 cupcakes; frost and fill 7” round layer cake; or frost 8” round layer cake that has a separate filling.
Swiss Meringue Buttercream
Equipment:
Heatproof bowl that fits over a saucepan (pic below)
Saucepan
Electric mixer
Instant read thermometer
Food scale or measuring cups
Measuring spoon
5 large egg whites (150 g)
1 1/2 cups sugar (300 g)
1/2 teaspoon + 1/8 teaspoon cream of tartar (total 5/8 teaspoon)
4 sticks unsalted butter (450 g) with butterfat 83%*
Cut butter into 2” cubes. Leave butter out to warm to 70°. Butter should be soft enough to give when gently pressed. But it should not be greasy looking or soft like whipped cream.
Place 3” of water in a saucepan pan. Bring to boil, then reduce heat to simmer.
Place egg whites and sugar in heatproof bowl. Whisk to combine.
Place bowl over saucepan and whisk slowly and continuously.
Heat egg whites to 160°. Remove from heat and begin beating immediately on medium high.
After one minute, add cream of tartar to egg whites. Eggs should be foamy and translucent (pic below).
Continue beating to stiff peaks (see pics in post above). This will take approximately 5 minutes on medium high speed.
While beating, add butter one cube at a time. Allow each cube to blend in before adding the next cube of butter. This will take about 5 minutes.
IMPORTANT NOTE: when butter is added to meringue, it will soften to a soup like consistency. This is normal. Emulsification is the next step. After the mixture is emulsified, it will thicken into a big fluffy mass.
After all the butter is added, reduce the mixer speed to low.
Continue beating on low until mixture forms a thick creamy mass. This will take 8 - 10 minutes.
The total beating time is 16 - 20 minutes:
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Set bowl on top of a pan of simmering water
Best egg whites until just foamy and still translucent, then add cream of tartar