Cream the butter until smooth.
Meanwhile, sift powdered sugar and heat-treated and cooled flour.
Add the sifted sugar-flour mixture in a few additions to the bowl of the stand mixer or to the mixing bowl if using a hand mixer.
Measure the maple syrup in a liquid measuring cup and when the powdered sugar and butter are combined and smooth, set the mixer to low speed and slowly add the syrup in a steady stream. Mix until incorporated and then add the extracts and salt and mix until incorporated.
Increase the mixer to medium-high speed and mix until the frosting lightens in color and increases in volume. Pause mixing and use a spatula to scrape down the sides of the bowl as needed.
This recipe will make enough frosting for one 9” cake or one 6” 3-layer cake. Store any extra frosting in an airtight container in the fridge for up to one week. To soften and smooth refrigerated buttercream, let it come to room temperature and mix again using stand or hand mixer until smooth.