In a medium saucepan, stir together chocolate chips and condensed milk. Place pan over the lowest heat possible on the stovetop, and place a heat-proof trivet or pot holder on the counter near the stove. Stir chocolate chip mixture constantly, scraping the bottom and sides the whole time, until the chocolate chips are melted. If you smell the slightest burning smell or think the chocolate might be getting too hot, move the pan to the trivet and stir it for about 10 seconds off the heat. Then, put it back on the burner and repeat. I moved it back and forth from the heat to the counter every 30 seconds or so once the chocolate started melting.