Preheat the oven to 350°F (175°C) and arrange oven-safe bowls on a baking sheet to use as molds; have a ladle or small bowl ready to press tortillas into the molds.
Heat 2 tablespoons of the oil in a large skillet over medium heat until shimmering (about 350°F if using a thermometer).
Carefully add one tortilla to the hot oil and fry 30–45 seconds on the first side until golden-brown in spots, then flip and fry 30–45 seconds on the second side; the tortilla should puff and develop deep golden spots.
Using tongs, immediately transfer the tortilla to an oven-safe bowl and press it into shape with a ladle or smaller bowl so it fits the mold; sprinkle with kosher salt if desired.
Repeat frying and shaping with the remaining tortillas, adding more oil to the skillet as needed between batches.
Place the shaped tortilla bowls (on the baking sheet) into the preheated oven and bake 12–14 minutes, or until they are crisp and golden brown.
Remove the shells from the oven, let them cool briefly, then lift them off the molds and cool completely on a wire rack before serving.