Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper or lightly oil it.
Lightly spray or brush both sides of each corn tortilla with cooking spray or vegetable oil.
If using, sprinkle a light dusting of chili powder on one or both sides of the tortillas.
Fold each tortilla in half over the long side and place them standing on edge on the prepared baking sheet so they form taco-shell shapes. If needed, use two baking sheets or space them to avoid touching.
Bake for about 8–12 minutes, until the tortillas are crisp and golden, flipping once halfway through if they are browning unevenly.
Remove from the oven and let the shells cool slightly so they firm up before filling.