Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper or foil.
Lightly coat both sides of each corn tortilla with cooking spray or brush with a thin layer of vegetable oil.
Sprinkle chili powder on the tortillas if using, to taste.
Arrange the tortillas on the prepared baking sheet. For U-shaped taco shells, drape tortillas over two parallel bars of the oven rack or over a folded foil support so they hold shape; alternatively, fold each tortilla in half and press lightly while baking to form shells.
Bake for about 8–12 minutes, flipping once with tongs halfway through, until the shells are crisp and golden at the edges.
Remove from the oven and let the shells cool a few minutes so they firm up, then fill as desired.
