Preheat the oven to 425 F and line a large baking sheet with aluminum foil.
In a medium light-coloured saucepan, melt the butter over medium heat. Continue cooking the butter, swirling the pan frequently, until it starts to turn golden and smell nutty, and brown bits have started to collect at the bottom of the pan. Remove from heat.
Immediately whisk in the maple syrup and red pepper flakes.
Toss the sweet potatoes with 1/2 of the browned butter glaze, a large pinch of salt and a few grinds of fresh pepper, and spread in one layer on the prepared baking sheet.
Roast the sweet potatoes, for about 40 minutes, tossing with a spatula once, about halfway through, until caramelized and tender.
Transfer the sweet potatoes to a bowl, pour the remaining brown butter maple glaze over them, sprinkle with parsley and additional salt if desired, and toss gently to coat.