Line an 8x8-inch pan with parchment paper and grease lightly, set aside.
Add pure maple syrup to a medium saucepan, heat to a boil.
Whisk in heavy whipping cream and attach a candy thermometer.
Heat over medium heat, whisking often, until thermometer reads 234-236 degrees F, about 17-18 minutes.
Immediately remove from heat and whisk in butter and salt, then let sit and cool for about 10 minutes or until under 200 degrees F and resembles a thick caramel sauce.
Whisk vigorously until thickened and creamy in appearance, and no longer like caramel sauce, about 5 minutes.
Pour into prepared pan, smooth with spatula.
Top with optional chopped pecans or walnuts.
Allow to cool to room temperature, about 1.5-2 hours, or cover and refrigerate for an hour.
Remove from pan using parchment paper, then cut into bite-sized squares and serve or package for gifting.