Here is a smbc lemon buttercream that I have made with a lemon cake.
1 cup sugar
4 large egg whites
3 sticks (12 ounces) unsalted butter, at room temperature
¼ cup fresh lemon juice (from 2 large lemons)
1 teaspoon pure vanilla extract
To Make the Buttercream
Put the sugar and egg whites in a mixer bowl or another large heatproof bowl, fit the bowl over a plan of simmering water and whisk constantly, keeping the mixture over the heat, until it feels hot to the touch, about 3 minutes.
The sugar should be dissolved, and the mixture will look like shiny marshmallow cream.
Remove the bowl from the heat.
Working with the whisk attachment or with a hand mixer, beat the meringue on medium speed until it is cool, about 5 minutes.
Switch to the paddle attachment if you have one, and add the butter a stick at a time, beating until smooth.
Once all the butter is in, beat in the buttercream on medium-high speed until it is thick and very smooth, 6-10 minutes.
During this time the buttercream may curdle or separate just keep beating and it will come together again.
On medium speed, gradually beat in the lemon juice, waiting until each addition is absorbed before adding more, and then the vanilla.
You should have a shiny smooth, velvety, pristine white buttercream. Press a piece of plastic against the surface of the buttercream and set aside briefly.