Preheat your oven to 180°C (350°F) and toast the pecans in a dry skillet until fragrant and slightly browned, then roughly chop them and set aside.
Place the sweet potatoes in a large pot and cover with salted water. Bring to a boil over high heat and cook until tender, about 15 minutes. You should be able to easily pierce them with a fork.
Drain the sweet potatoes thoroughly and transfer them to a large mixing bowl. Mash them with a potato masher or fork until smooth and creamy, with no lumps remaining.
Stir in the unsalted butter until melted and incorporated. Add a splash of bourbon and sprinkle in the cinnamon, mixing well to combine all the flavors.
Spread the sweet potato mixture evenly into a buttered baking dish, smoothing the top with a spatula for an even surface.
Sprinkle the mini marshmallows generously over the surface, followed by the toasted pecans, creating a layered topping that will melt and crisp during baking.
Bake the casserole in the preheated oven for 20-25 minutes, or until the marshmallows are golden brown and bubbling around the edges.
Remove from the oven and let it sit for a few minutes to settle. Serve warm, enjoying the creamy sweet potatoes with the crunchy topping and warm bourbon aroma.