Heat a grill or grill pan to medium-high heat. Grill the poblanos skin-side down until blistering, about 5 minutes. Remove from the heat and let cool slightly.
While the poblanos are cooling, spray the grill pan with nonstick cooking spray and grill the Cotija cheese on all sides until browned, 2 to 3 minutes. Remove from the heat and let cool. Cube the cheese and reserve for garnish.
Remove and discard the blistered skin from the poblanos. Dice 1 poblano and reserve for garnish. Roughly chop the remaining 2 poblanos.
Melt 1 tablespoon of the butter in a medium saucepan over medium heat. Add the onions and the 2 chopped poblanos and cook, stirring occasionally, until tender, 5 to 10 minutes. Stir in the Jack cheese. Pour in the 1 quart chicken broth and bring to a rolling boil. Add the cilantro. Boil for 30 seconds and then remove from the heat. Carefully puree until smooth with a handheld immersion blender or in 2 to 3 batches in a blender (do not fill the blender more than halfway full per batch). Stir in the remaining 1 tablespoon butter.
Make a cornstarch slurry by whisking together the cornstarch with 4 teaspoons cold water or chicken broth in a small bowl. Stir the slurry into the soup to thicken. Season with salt.
Garnish with the pepitas, diced poblano and grilled Cotija cheese.