Heat condensed milk in a cast aluminum or iron 2 quart [2 lt] pot over medium-high heat.
Once the milk breaks the boil it will become thinner, this is normal, start stirring with a wire whisk to prevent it from sticking to the bottom.
Once the milk has become thick enough to start lifting from the bottom (10 to 15 minutes) stir in sugar.
Continue whisking vigorously until it has become so thick that it sticks to the wire whisk (similar consistency to peanut butter.
Spoon into the mold and smooth out. Let it cool to room temperature.
Remove from the mold and cut into 6 squares.
Notes
I did not have a mold the size of the traditional Dominican one, I improvised by covered a cardboard box of similar size with aluminum foil and spraying with oil. The size I used was 3"L x 4 ½"W x 2"H [6.75 cm L x 9.5 cm W x 4.5cm].