Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
- Prepare potatoes: Boil, steam, or bake sweet potatoes until completely tender. Peel while warm and mash thoroughly until perfectly smooth. Measure mashed potato volume and transfer to a large bowl.
- To the mashed potatoes, add the melted butter, granulated sugar, brown sugar, eggs, milk/cream, vanilla, cinnamon, nutmeg, and salt. Beat until just combined and homogenous. Pour this mixture evenly into the prepared baking dish.
- For the topping: Melt the 1/2 cup of cold butter in a small saucepan over medium heat until the milk solids turn golden brown and nutty. Immediately pour the brown butter into a heatproof bowl to stop cooking and let cool slightly.
- In a separate bowl, whisk together the flour, brown sugar, and salt for the topping. Pour the cooled brown butter over the dry ingredients and use a fork to mix until large, crumbly clumps form. Gently fold in the chopped pecans.
- Sprinkle the streusel mixture evenly over the sweet potato base.
- Bake for 35–40 minutes, or until the topping is deep golden brown and the casserole filling is set.
- Allow the casserole to rest on a cooling rack for at least 15 minutes before slicing and serving.