Using all salted butter in buttercream can result in a frosting that is too salty, especially if the recipe also calls for a significant amount of powdered sugar. Salted butter already contains added salt, and when combined with the powdered sugar, the saltiness can become quite pronounced and overpower the sweetness.
Most traditional buttercream recipes use unsalted butter as the base to give the baker more control over the amount of salt added to the frosting. By using unsalted butter, you can adjust the level of saltiness to achieve a balanced and desired flavor profile.
If you only have salted butter on hand and want to use it for buttercream, you can try reducing or eliminating any additional salt called for in the recipe. This way, you can temper the saltiness to some extent. However, it's essential to taste the frosting as you go and make adjustments as needed to avoid an overly salty result.
For the best results and the most control over the flavor, using unsalted butter is recommended when making buttercream. If you prefer a hint of salt to balance out the sweetness, you can add a pinch of salt to the frosting gradually, tasting as you go, until you achieve the desired balance.