Slice off about 1/2-inch from the ends of the potatoes. Then cut a thin slice the length of one side, to provide a stable platform while you finish cutting the potato. Now slice the potato lengthwise into 1/2-inch-thick planks, then cut those planks lengthwise into 1/2-inch-wide sticks. Place these fries in a bowl of ice water and agitate them to remove the starch. Rinse, then put in another ice-water bath and agitate again until the water is clear (which means there's no more starch). Spread the fries on a clean dish towel and pat dry.
First fry: Pour peanut oil into a cast-iron pot—it should be about 3/4 full. Heat oil to 325 degrees F. Fry in batches until the corners just start to color, about 3–5 minutes. (This "oil blanching" cooks them through.) Remove and let cool to room temperature.
Second fry: Heat oil to 375 degrees F. Fry in batches until golden brown and crispy, about 2–3 minutes. Drain on paper towels and season with kosher salt. Serve with ketchup spiked with hot sauce.