In a medium bowl, toss Monterey Jack and cheddar cheese with cornstarch until evenly coated. Transfer cheeses to a medium saucepan and add evaporated milk and hot sauce. Place over low heat and cook, whisking constantly, until melted and thickened, about 5 minutes. Let stand until cooled to room temperature.

Arrange jalapeño rings on a parchment-lined baking sheet. Spoon cheese sauce into jalapeño rings, filling about ⅔ full and patting down with fingers.

Place cornflakes and breadcrumbs in the work bowl of a food processor and pulse until cornflakes are finely processed. Transfer breading mixture to a shallow bowl.

Place buttermilk, flour, and eggs in separate shallow bowls. Working one at a time, dunk a jalapeño slice into buttermilk and then roll around in flour until completely coated. Dunk jalapeño slice into eggs and then roll around in breadcrumb mixture until completely coated. Transfer to a wire rack set in a rimmed baking sheet. Repeat with all jalapeño rings. Transfer baking sheet to freezer and freeze until firm, about 30 minutes.

Preheat oven to 425°F (220°C). Remove baking sheet from freezer. Brush breaded jalapeños all over with bacon fat or oil. Transfer baking sheet to oven and cook until cheese is heated throughout and outsides are browned and crisp, 15-18 minutes. Remove from oven and transfer jalapeño poppers to a tray. Season with salt. Serve immediately with sauce of your choice.