Start by making the filling. Whisk together the pumpkin and butter until well combined. Then whisk in the brown sugar, milk, eggs, spices, and vanilla until very well combined.
Roll out the pie dough and shape into the pie pan. Brush the edges with egg wash. Pour in the filling. Bake the pie for 40 minutes.
While it’s baking, make the streusel by mixing everything together with your hands. After 40 minutes of baking, spoon the streusel on top of the pie and bake another 20 minutes…until the pie is set…when a butter knife into the pie comes out clean.