Cook the pasta. Bring a large pot of water to a rapid boil, then season generously with a few tablespoons of Kosher salt. Add 8oz pasta, then cook according to package directions until al dente (or 1 minute more, depending on your preference). Reserve about ½ cup of cooking water, then drain pasta in a colander.
Sauté vegetables. While the pasta cooks, heat 2 Tbsp olive oil and 2 Tbsp butter in a large skillet over medium-high heat. When hot, add ½ cup diced red onion, the white parts of the scallion (reserve the green part), and chopped jalapeño. Season with ½ tsp kosher salt then cook, stirring occasionally, until softened, about 3-5 minutes.
Sauté corn. Add corn kernels, season with remaining ½ tsp kosher salt, stir well, then cook for an additional 6-8 minutes, or until the corn begins to blister and take on a golden color. Stir in 2 chopped cloves of garlic, then cook 1 minute more.
Make the dressing. In a large mixing bowl, whisk together ½ cup mayonnaise, ¼ cup sour cream, 2 Tbsp lime juice, 2 tsp chili powder, and ¼ tsp cayenne pepper. If you want the dressing less creamy, add a splash of reserved pasta water.
Toss together. To the pasta salad dressing, add cooked pasta, the sautéed corn mixture, reserved green onions, 2 oz cotija, and ½ cup chopped cilantro. Toss to evenly coat, then taste and adjust seasoning as needed.
Serve! Transfer to a large serving bowl, then crumble a little extra cotija on top. Sprinkle with extra chili powder and fresh cilantro leaves, then enjoy!