Sift the sugar. Add the powdered sugar to a mixing bowl and sift to remove lumps. This ensures a smooth, glossy finish.
Stir in maple syrup. Pour the maple syrup over the sugar and whisk until thick and paste-like. It will look too thick at first—this is right.
Add butter and flavorings. Whisk in the melted butter, vanilla, and salt.
If you love bold maple flavor, add the maple extract now.
Thin to the right consistency. Add milk 1 tablespoon at a time, whisking until the glaze is silky and flows in a ribbon. Target consistency: It should coat the back of a spoon and slowly drip off. For dipping, aim slightly thicker; for drizzling, a touch thinner.
Adjust sweetness and balance. Taste and add a pinch more salt if it’s too sweet, or a teaspoon more maple syrup if you want deeper flavor. If it gets too thin, whisk in a tablespoon of powdered sugar.
Glaze warm donuts. Dip freshly fried or baked donuts that have cooled for 5–10 minutes.
Dip the top side, let excess drip back into the bowl, then flip right-side-up on a wire rack set over a tray.
Let it set. Leave the donuts on the rack for 15–20 minutes. The glaze will firm up to a smooth, shiny finish you can touch without smearing.
Serve or store. Enjoy right away, or store as noted below. If the glaze thickens while you work, whisk in 1 teaspoon of milk to loosen it.