Combine the sweetened condensed milk, chocolate chips, and butter in a microwave-safe bowl. Heat in 30-second bursts, stirring well each time, until melted and glossy, about 1 1/2 minutes.

Scrape the mixture into an 8-inch square pan lined with parchment and spread into an even layer. Tap the pan on the counter a few times to settle the surface.

Chill for at least 2 hours until firm. Lift out the parchment, peel away, and cut into squares. Serve at cool room temperature.