Preheat oven to 400 degrees. Prick sweet potatoes a few times with a fork. Bake on a foil-lined baking sheet until tender, 1 hour 15 minutes. Let cool slightly.
Scoop out flesh from sweet potatoes, and puree in a food processor with 1 1/2 teaspoons salt until smooth (you may need to do this in 2 batches). Season with pepper.
Heat a saucepan over medium heat. Add butter; cook, swirling, until deep brown with a nutty aroma. Remove from heat; swirl in syrup. Reheat puree; transfer to a bowl. Pour butter over top; sprinkle with pecans.