In a small saucepan over medium-high heat, melt butter. Add sugar and salt.
Cook, stirring constantly, until sugar is completely dissolved, then adjust heat to medium and boil 2 minutes longer.
Add maple syrup and boil, stirring frequently, until sauce is thick, smooth, and coats a spoon, 2 to 4 minutes longer.
Remove from heat and hold warm for serving. Pour over ice cream, pound cake, oatmeal, or fruit crisps or use as dipping sauce for fresh fruit.