Step 1: In a medium heatproof bowl, empty the flavored Jell-O powder. Pour 1 cup of boiling water over the Jell-O powder and whisk continuously for 2 minutes until the powder is completely dissolved.
Step 2: In a small separate bowl, sprinkle 1 packet of unflavored gelatin over 1/4 cup cold water. Let it sit for 2-3 minutes to bloom. Microwave for 10-15 seconds until clear and dissolved. Add this dissolved gelatin to your hot flavored Jell-O mixture and stir well to combine.
Step 3: Lightly grease a 9x13 inch baking pan. Pour the Jell-O mixture evenly into the prepared pan. Carefully transfer the pan to the refrigerator and chill for 20-30 minutes, or until the Jell-O is partially set (jiggly but not completely firm).
Step 4: While the Jell-O chills, prepare the white layer. In a medium bowl, sprinkle 1 packet of unflavored gelatin over 1/2 cup cold water. Let it bloom for 2-3 minutes. Microwave for 15-20 seconds until clear and dissolved.
Step 5: In another bowl, combine the can of sweetened condensed milk with the dissolved gelatin. Whisk until thoroughly combined and smooth.
Step 6: Gently and evenly pour the sweetened condensed milk mixture over the partially set Jell-O layer in the pan. Return the pan to the refrigerator and chill for at least 3-4 hours, or preferably overnight, until both layers are very firm.
Step 7: Once completely set, remove the pan from the refrigerator. Run a thin knife around the edges of the pan to loosen the gelatin. Dip the bottom of the pan briefly in warm water (about 10 seconds) to help release it. Place a large piece of parchment paper or a cutting board over the pan and invert it to release the gelatin sheet.
Step 8: Cut the large gelatin sheet into strips, approximately 1 inch wide, using a sharp knife or pizza cutter.
Step 9: Starting from one end, carefully and tightly roll up each strip into a spiral. Place the finished Jello Roll Ups seam-side down on a serving platter. Chill again if needed before serving.