Preheat your oven to 325ºF. Lightly spray an 8×8-inch or 9×9-inch pan with cooking spray and set aside.
Place sweet potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook 10 to 15 minutes or until tender. Drain potatoes and place in a large bowl.
Add heavy cream, eggs, ¼ cup brown sugar, 1 Tbsp vanilla, pumpkin pie spice to cooked sweet potatoes. Mix with a hand-held mixer until smooth. Fold in pecans.
Spread potato mixture into prepared pan.
Combine ⅓ cup brown sugar, sweetened condensed milk, melted butter, and ½ tsp vanilla; spoon over potato mixture. Cut through with a knife to swirl the sauce.
Bake, uncovered, for 30 to 40 minutes.