Pour maple sap into a large stockpot and bring to a gentle boil over medium heat.
Reduce heat to medium-low and simmer, stirring occasionally, until the liquid reduces by about half, forming a light syrup.
Gradually stir in granulated sugar, ensuring it dissolves completely and mixture remains smooth.
Attach a candy thermometer and continue simmering until the syrup reaches 219–220°F (104–105°C).
Skim any foam from the surface with a spoon to keep the syrup clear and glossy.
Remove from heat and stir in vanilla extract and salt, if using.
Pour syrup through a fine mesh strainer or cheesecloth into sterilized jars while hot.
Leave a small headspace (about 1/4 inch) in jars to allow for expansion.
Let syrup cool completely at room temperature; it will thicken slightly as it cools.
Store in the refrigerator for up to 6 months or process in a boiling water bath for shelf-stable storage.
