Beat butter for 1 minute with whisk attachment.
Combine powdered sugar with meringue powder, salt, and any other dry ingredients you might be using, such as espresso powder, freeze dried fruit powder, etc. With the mixer running on low, add the dry ingredients to the butter by spoonfuls and mix on low to combine. It will be clumpy at this point.
Add vanilla, along with any other wet ingredients you're adding, like jam or fruit reductions or gel food coloring. Wait to add the milk until you need it for desired consistency.
Increase speed to medium high and whip for 4-5 minutes until impossibly light and fluffy, scraping the bowl several times, and adding the milk if needed. Turn the mixer down to low and mix for 1 minute - this helps to eliminate some of the bigger air bubbles throughout the buttercream if you're trying to frost your cake with a smooth finish.
If using any add-ins, such as chopped chocolate or nuts, fold them in with a spatula after whipping the buttercream. Be careful of adding whole pieces of fruit or anything that contains water, as it will dissolve the sugar and turn the buttercream too liquidy.