Preheat the oven to 425 degrees. Peel the potatoes, and cut into bite-sized pieces. Put the potatoes in a pot, add cold water to cover, and swirl the potatoes around with your hand to remove excess starch. Drain, then cover the potatoes with cold water, cover, put on the stove, and bring to a boil over high heat. Remove the cover, turn the heat down to medium high, and boil the potatoes until just barely tender, 5-8 minutes.
Drain the potatoes in a colander. Add the kosher salt, and toss with the salt. Transfer the potatoes to a bowl, add the olive oil, and toss to coat. Put the potatoes on a baking sheet (I lined mine with a Silpat so they didn't stick, but you could use parchment paper, or nothing). Put in the oven, and roast, turning and tossing occasionally, until crispy and golden brown, around 40-50 minutes.
Meanwhile, make the bravas sauce: put the ketchup, sriracha, and pimentón (if using) in a small bowl, and stir to combine.
When the potatoes are done, transfer them to a platter, add a little kosher salt, and serve with the bravas sauce on the side, or drizzled over the top.
Don't crowd the potatoes: Otherwise, they'll steam rather than become crispy while cooking.
For crispier potatoes: Boil them with ½ tsp baking soda. This draws starch to the surface for crispier edges. Also, fluff up the edges when tossing them with salt and oil. This creates ridges to crisp up.
Ensure the potatoes are dry: Let them steam dry or pat dry before tossing them with oil/salt. Excess water will steam the potatoes instead of making them into crispy patatas bravas.
Tweak the sauce: Adjust the ratio of ingredients in the bravas sauce recipe to tweak the spiciness — no matter whether you make the quick 'cheat' version or the simmered one.