01 - Preheat oven to 375°F. Generously butter a 12-inch cast iron skillet or a medium casserole dish and set aside.
02 - In a large bowl, whisk pumpkin puree, granulated sugar, eggs, pumpkin pie spice, salt, and vanilla extract until well blended. Whisk in heavy cream until the mixture is smooth. Pour evenly into the prepared skillet or casserole dish and set aside.
03 - In a medium bowl, stir together flour, granulated sugar, cinnamon, and salt. Pour in melted butter and mix with a fork until coarse crumbs form. The mixture should resemble a crumbly streusel.
04 - Spread the cinnamon streusel evenly over the pumpkin mixture. Bake for 40 to 45 minutes, or until the pumpkin layer is set and the streusel is golden brown. If the topping browns too quickly, tent with foil to prevent over-browning.
05 - Remove from oven and allow to cool for 10 minutes for the filling to firm up. Serve warm, accompanied by ice cream or whipped cream if desired.