In a stand mixer fitted with the whisk attachment {the paddle attachment works too}, whip the butter for 30 seconds.
Add the powdered sugar 1 cup at a time. Scrape the sides in between each addition and start mixing slow then increase to medium/high. Mix for 30 seconds between each sugar addition.
{3 cups for plain frosting or 3.5 cups for maple buttercream}
Stop and scrape down sides. Add the maple syrup now if doing so.
{1-2 tsp of vanilla paste or extract can be added for vanilla buttercream frosting}
Mix on medium/high speed for 3-4 minutes. Frosting will be light and creamy.
Chill in the refrigerator for 5 minutes before frosting your cake.
Store leftover frosting in an airtight container in the refrigerator.
Notes
If making plain buttercream, heavy cream can be added to get the desired consistency. Add 1-2 Tb, 1 Tb at a time. Yields approx 4 cups.