Preheat your grill to medium heat. Grill the corn and poblano pepper for 15 minutes. The pepper should be charred and the corn should be tender with some grill marks. Remove from the grill and let the corn and pepper cool to room temperature. When the corn is cool enough to handle, using the corn stripper, remove the kernels off the cob. You'll also want to remove the skin off the poblano pepper, slice it in half, and remove the seeds. Set both corn and poblano pepper to the side.
In a large soup pot, heat oil and butter over medium heat. Add onion and garlic. Cook until tender, about 5 minutes. Add the cumin and cook until fragrant about 1 minute. Stir in 1-1/2 cups of grilled corn kernels, vegetable stock, and peeled and chopped potato. Mix well and bring to a boil then reduce heat. Simmer, covered, for 10-15 minutes until the potato is cooked (stir occasionally).
In a bowl add the flour, salt, and pepper and mix. Then stir in the whole milk into the flour mixture. Stir until smooth.
Gradually stir the whole milk mixture into the soup as well as half of the poblano pepper. Cook for five minutes, stirring until slightly thick and bubbly. Using an immersion blender, puree the mixture to desired consistency. Ladle soup into bowls, top with remaining grilled corn kernels, chopped poblano pepper, and cilantro.