Preheat oven to 375 F and generously butter a 9 X 9 inch baking dish.
In a large mixing bowl, whisk together the pumpkin puree, 1 cup granulated sugar, eggs, pumpkin pie spice, 1/2 tsp salt, vanilla extract until completely smooth.
Whisk in the heavy cream until fully incorporated and silky, then pour the custard base into the baking dish.
In a separate medium bowl, whisk together the flour, 1 1/2 cups sugar, cinnamon and remaining 1/2 tsp salt until uniform.
Pour in the melted butter in three separate additions, tossing and pinching the mixture together with a fork or your fingers until thick, buttery crumbs form.
Scatter the cinnamon streusel mixture evenly across the top of the pumpkin custard layer covering it completely.
Bake for 40 to 45 minutes or until the spiced pumpkin custard is gently set and the topping turns a deep golden brown. If the top browns too fast, loosely tent the dish with aluminum foil for the final stretch.
Remove from the oven and allow the crisp to rest for about 10 minutes so the custard base can firm up slightly, before spooning into bowls warm with a scoop of vanilla ice cream or a cloud of fresh hipped cream.