In a saucepan heat the oil over medium heat, then add the butter. Melt the butter, stirring to combine the oil and butter.
Place the serrano pepper halves into the saucepan and saute them in the oil/butter mix for 1 minute, stirring often to infuse the pepper oils into the butter and oil.
Add the pinto beans and low sodium chicken broth to the saucepan. Continue to cook, stirring the beans often and mashing them smooth in the process, for approximately 10 minutes.
Remove the serrano pepper halves from the beans, then add salt and pepper to taste.
Place the refried beans into a bowl and allow them to cool for 5 minutes. Top the beans with shredded cheese and serve.
If no serrano peppers are at hand, you can opt for jalapeño peppers. They are half as spicy as serranos, though, so increase the amount used to 3 or 4. Or, if you prefer a lesser spice, keep the amounts equal (2 jalapeño peppers). Optional: You can chop up the serrano pepper and stir it into the refried beans for extra spiciness once the beans have been cooked in the pepper oils.