Step 1

Preheat oven to 450°. Place potatoes in a large pot and pour in cold water to cover; season with salt. Bring to a boil; reduce heat and simmer until tender, 20–25 minutes. Drain and return to pot to dry out; let cool. Transfer to a rimmed baking sheet and flatten with your hands or the bottom of a mug. You want the skins to tear to expose some flesh; some may fall apart.

Step 2

Meanwhile, heat ¼ cup vegetable oil in a medium skillet over medium-high. Cook onion, stirring occasionally, until soft, about 5 minutes. Add chorizo; cook, breaking up with a spoon, until browned and cooked through, 5–7 minutes. Transfer to a fine-mesh sieve set over a measuring cup. Drain, pressing on solids, until you have about ¼ cup chorizo oil. Add ½ cup vegetable oil to chorizo oil to make ¾ cup total; set chorizo mixture aside separately.

Step 3

Whisk egg yolk, garlic, and 1 tsp. water in a medium bowl to combine. Whisking constantly, gradually drizzle in chorizo oil, drop by drop at first; keep whisking until aioli is thickened and smooth. Whisk in lemon juice; season with salt.

Step 4

Drizzle potatoes with remaining 2 Tbsp. vegetable oil and turn to coat; season with salt. Roast, tossing once, until golden and crisp on outer edges, 20–25 minutes; season with salt.

Step 5

Toss potatoes, half of aioli, half of scallions, and reserved chorizo mixture in a large bowl; season with salt if needed.

Step 6

Spread remaining aioli on a platter and arrange potato salad on top. Sprinkle with remaining scallions, then ancho chile powder, if using. Serve with hot sauce.

Step 7

Do Ahead: Potatoes can be boiled and smashed 1 day ahead; cover and chill. Chorizo mayonnaise can be made 1 day ahead; cover and chill.