Set your oven to 350°F and let it fully heat up. A hot oven helps the crust bake evenly from the start. I also slide the rack to the middle so the pies cook gently.
Unroll the pie crust on a clean, lightly floured surface. Use a 4-inch cutter or a jar lid to cut 8 to 9 circles. Gather the scraps, re-roll them, and cut again until the dough is used up.
Spray each muffin cup with nonstick spray. Press one dough circle gently into each cup, so it hugs the bottom and sides. Tuck the edges up neatly, and a mini muffin tin works too if you cut smaller circles.
In a mixing bowl, add the pumpkin puree, sweetened condensed milk, egg, salt, and pumpkin pie spice. Whisk until the mixture looks smooth and glossy. Take your time here because no streaks should remain.
Spoon the filling evenly into the crust-lined cups. Fill them close to the top, since this filling barely puffs. Bake for 25 to 35 minutes until the centers are set and the crust edges turn golden.
Let the pies cool completely in the pan. Cooling lets the filling firm up so it does not crumble. Run a butter knife around each pie, then lift it out gently.