Preheat your oven to 375°F (190°C) and butter a 12-inch cast iron skillet or medium casserole dish.

In a large mixing bowl, combine the mashed sweet potatoes, granulated sugar, eggs, vanilla extract, pumpkin pie spice, and a pinch of salt. Whisk for 2-3 minutes until smooth.

Gradually stir in the heavy cream until the filling is creamy.

Pour the sweet potato filling into the prepared dish, spreading it evenly with a spatula.

In a separate bowl, mix together the all-purpose flour, sugar, cinnamon, and a pinch of salt. Add melted butter and combine until crumbly.

Sprinkle the streusel topping evenly over the sweet potato filling.

Bake for 40-45 minutes until the filling is set and the topping is golden brown.

Let cool for about 10 minutes before serving.