Add the sweetened condensed milk and butter to a sauce pan. Heat to medium. When the butter melts, add the cinnamon. Stir with a wooden spoon to combine.
When it begins to boil, reduce heat slightly and continue cooking at a steady boil for 7-10 minutes or until the mixture comes away from the sides completely when you tilt the pan, remove from heat.
Place into a greased bowl Cover and let cool for 30-35 minutes. When ready to roll, apply a light coat of oil to hands and roll the candy into balls. Gently roll in sugar/cinnamon mixture.
If using the pecan halves, gently press a pecan into the center of each bolita de leche. Let sit at room temperature for a few hours before storing in an airtight container.
Notes
If your cooled dulce de leche gets too hard when cooled, pop it in the microwave for 35-45 seconds to soften the candy. Roll as instructed.