With the mixer on low, gradually add all but 1 cup (4 ounces) of the confectioners' sugar, 3 tablespoons whole milk, 2 teaspoons pure vanilla extract, and a pinch of kosher salt. Once combined, increase the mixer to medium-high and beat for 3 to 5 minutes, or until the buttercream is white, fluffy, and smooth. Reduce the mixer to low and add the remaining 1 cup of confectioner's sugar as needed 1/4 cup at a time, until the desired consistency is reached. The buttercream should be soft and spreadable, but not runny.