In a heavy-duty saucepan over medium heat, combine the sweeteners and water, whisking until dissolved. Bring the mixture to a boil, then reduce the heat to medium-low and simmer for 5 minutes, ensuring it doesn't burn.
Add butter, whisk to melt, and continue simmering until the mixture thickens and darkens slightly (about 2 minutes).
Remove from heat, whisk in the maple or vanilla extract and salt (if using), and allow it to cool for 10-15 minutes before serving.
Transfer the syrup to a glass jar for storage in the refrigerator.