Prepare Your Workspace: Wipe down your mixing bowl and whisk with lemon juice or vinegar to remove any oils or traces of fat.
Combine Egg Whites, Sugar, and Water: Place egg whites, sugar, salt, and water into a clean, heatproof mixing bowl. Whisk gently to combine.
Heat the Mixture: Set the bowl over a pot of gently simmering water (double boiler method). Whisk constantly until the sugar dissolves completely and the mixture reaches at least 160°F (71°C).
Whip to Meringue: Transfer the bowl to a stand mixer fitted with a whisk attachment. Beat the warm mixture on high speed until stiff, glossy peaks form and the bowl is cool to the touch.
Add Butter Gradually: Switch to the paddle attachment. Reduce mixer speed to medium. Add softened butter a few tablespoons at a time, allowing each portion to fully blend before adding more.
Flavor and Finish: Add preferred flavorings and beat briefly to combine. Taste and adjust seasoning if needed.
Use or Store: Your finished buttercream is ready to spread, pipe, or refrigerate. Store in an airtight container up to 5 days, or freeze for longer storage.
