Shuck the corn. Stand each ear up in a bowl and use a knife to scrape off the kernels.

Put the corncobs and 2 cups water in a large pot with a tight-fitting lid over medium-high heat. Sprinkle with salt and pepper. Bring to a boil, then lower the heat so the water bubbles gently, cover, and cook, checking occasionally, for about 30 minutes. Leave the cobs in the pot until you are ready to make the soup, then discard them and save the corncob broth.

Put the butter or oil in a large, deep pot over medium-high heat. When the butter is melted or the oil is hot, add the onion and cook, stirring occasionally, until soft, about 1 minute. Lower the heat to medium and stir in the flour.

Cook, stirring constantly with a whisk or a wooden spoon, until the mixture starts to turn golden and the flour no longer smells raw, just a couple of minutes. Add the milk and the reserved corncob broth and raise the heat to medium-high. Stir or whisk constantly until the flour is dissolved and the soup starts to thicken, about 2 minutes.

Stir in the cayenne pepper, corn kernels, diced potatoes and bacon and bring to a boil, then lower the heat so that the soup bubbles gently.

Cook, stirring occasionally, until the potato is tender and the soup has thickened, 10 to 15 minutes. Taste, adjust the seasoning, and serve.