Cut each potato in half lengthwise, then cut each half lengthwise into 4 even wedges, so each potato gives 8 wedges.
Put the wedges in a large bowl, cover with cold water and soak for 30 minutes. Towards the end of soaking, set an oven rack in the lower-middle position and heat the oven to 425°F / 220°C. Line a large rimmed sheet pan with parchment paper.
Drain the wedges and spread them on clean kitchen towels. Pat them very dry on all sides; any water left on the surface will steam instead of crisp. Dry the bowl too.
In a small bowl, mix the cornstarch, salt, smoked paprika, garlic powder and pepper. Return the wedges to the dry bowl, sprinkle over the seasoning mix and toss until every wedge is lightly and evenly coated.
Drizzle with the olive oil and toss again until the wedges look glossy and no dry powder is left.
Arrange the wedges on the sheet pan with a cut side down and a little space between each one. Use two sheets if they are crowded.
Bake for 20 minutes, until the undersides are golden and release easily from the parchment. Flip each wedge onto its other cut side.
Bake for another 12 to 15 minutes, until both cut sides are deep golden and crisp and a knife slides easily into the thickest part.
Sprinkle with a pinch of salt and the parsley, if using, and serve straight away.