Preheat oven to 400 degrees F. Place your unbaked pie crust on a baking sheet lined with foil.
In a large bowl, whisk together the mashed sweet potatoes and beaten eggs until smooth. Add both sugars, salt, cinnamon, and pumpkin pie spice. Mix well. Stir in the light cream (or half-and-half) until silky.
Pour the filling into the prepared crust. Bake for 45–55 minutes, or until a knife inserted halfway between the center and edge comes out clean. The center should look set but still have a slight jiggle.
Let the pie cool completely on a wire rack before adding the topping.
In a small bowl, mix together the softened butter, brown sugar, and chopped pecans.
Gently spoon the mixture over the cooled pie (no need to spread perfectly).
Place the pie 5 inches below the broiler and broil for about 3 minutes, until the topping is bubbling and glossy. Cool completely again before slicing.