01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish and set aside.
02 - Place sweet potatoes in a large pot and cover with cold water. Bring to a boil, then reduce heat and simmer until sweet potatoes are fork-tender, about 15 to 18 minutes. Drain thoroughly.
03 - While sweet potatoes cook, melt 6 tablespoons butter in a small saucepan over medium heat. Stir and watch carefully as the butter foams and becomes deep golden with a nutty aroma, about 4 minutes. Remove from heat immediately.
04 - Using a potato masher or mixer, mash drained sweet potatoes in a large bowl until smooth. Add browned butter, maple syrup, bourbon, milk, eggs, vanilla, cinnamon, nutmeg, and salt. Mix until creamy and well blended.
05 - Spread the sweet potato mixture evenly into the prepared baking dish, smoothing the surface.
06 - In a medium mixing bowl, combine chopped pecans, brown sugar, oats, flour, cinnamon, and salt. Add melted butter and stir until the mixture is evenly moistened and crumbly.
07 - Scatter the pecan topping evenly over the sweet potato layer.
08 - Bake uncovered for 30 to 35 minutes, until the topping is golden brown and the casserole is heated through. Allow to rest for 10 minutes before serving.