In a large bowl, mix pumpkin puree, apple butter, and brown sugar together. Add in all spices, and mix until combined.
Add in half & half and eggs, then mix until fully incorporated.
Pour filling into prepared pie crust, and smooth the filling until it's in a nice, even layer.
Bake for 30 minutes.
While the pie is baking, prepare the brown sugar streusel.
In a small bowl, mix brown sugar, flour, and pecans together. With a pastry cutter or fork, work the butter into the brown sugar and flour mixture, until the streusel topping resembles coarse crumbs.
Sprinkle the streusel over the pie, and bake for an additional 30-35 minutes, or until the pie is set when jiggled slightly (or insert a knife into the center of the pie; if it comes out clean, the pie is done).
Allow to cool at least 30 minutes before serving (or refrigerate it).
Top with whipped cream.
Refrigerate leftovers.