Preheat oven to 375°F. Lightly butter a 12-inch cast iron skillet or 9×9-inch baking dish.
In a large bowl, whisk together pumpkin puree, sugar, eggs, pumpkin pie spice, salt, and vanilla. Add heavy cream and whisk until smooth.
Pour pumpkin mixture into the prepared baking dish and spread evenly.
In a separate bowl, combine flour, sugar, cinnamon, and salt. Gradually add melted butter, stirring with a fork until coarse crumbs form.
Sprinkle the cinnamon streusel evenly over the pumpkin filling without pressing it down.
Bake for 40–45 minutes, until the topping is golden brown and the pumpkin custard is mostly set. Cover loosely with foil if the topping browns too quickly.
Let cool for at least 10 minutes before serving warm with vanilla ice cream or whip cream.