To prepare enough butter cream to frost a two layer 9-in. round cake (or a three layer 8-in. round cake), you'll need:
- one pound (450 g) unsalted butter,
- four large eggs,
- 1 cup (200 g) granulated sugar,
- 1/2 teaspoon (3 g) table salt, and
- 2 tsp. pure vanilla extract.
- I used 2 whole passion fruits, I love the seeds you may prefer to strain them out.
Start by cracking all four eggs into the metal bowl of a stand mixer. Add the sugar, salt, and vanilla extract to the bowl.
Whisk until the eggs, sugar, salt, and extract are evenly combined.
Find a pot that the mixing bowl can sit on without touching the bottom of the pan. Pour about 1/2-in. (about 1 cm) water into the pot and bring to a boil. Reduce heat to a simmer and place the mixing bowl over the water pot to form a double boiler. This will allow us to heat the eggs slowly, minimizing the chances that the egg proteins will tighten up (forming scrambled eggs). Whisk continuously over the steaming water until the eggs reach 160°F / 71.1 Degrees Celcuis, as measured by an instant read thermometer.
Once the mixture reaches 160°F / 71.1 Degrees Celcuis, take it off the heat and beat on medium-high with the flat beater attachment for five minutes. The egg mixture will turn light yellow and fluffy. The mixture should have cooled by this time. Touch the side of the mixing bowl to check the temperature. If the mixture is too warm, it will melt the butter while you add it and the buttercream might not come together.
Reduce the mixer's speed to low and begin cutting pieces of butter into the mixer, waiting for ten to fifteen seconds before adding the next piece. One pound of butter should be cut into about 16 to 20 pieces (about 2 Tbs. per piece). During this process, the mixture will become lumpy, but don't worry about it, continue to cut pieces of butter into the mix until all the butter has been incorporated. At this stage I stirred in the pulp and juice of two passion fruits.