In a microwave-safe bowl, melt the chocolate chips on high for one minute. Stir, then microwave again for 30 seconds. Be sure to check and stir the chocolate as it melts. Chips will hold their shape and not melt unless stirred., Add the condensed milk and stir until it is all incorporated into the chocolate. The mixture will thicken up a little as you stir. This is normal., Cut the cold butter into small pieces, about the size of playing dice. Top the chocolate mixture with the butter and return to the microwave for another 30 to 45 seconds, until the butter is melted. Stir well., Finally, add the vanilla, stir until smooth and pour into a buttered 9" x 5" loaf pan., Chill for at least 2 hours before cutting and serving. Store at room temperature or can be frozen for up to 4 weeks.