Toss cheese and cornstarch in a medium heatproof bowl. Pour evaporated milk over and cook in microwave on high 1 minute. Whisk and continue to cook in 30-second increments, whisking in between, until melted and smooth (about 2 minutes total). Once cheese has melted, stir in Rotel and hot sauce, if using, and microwave an additional 30 seconds. Stir to combine.