Step 1: Prepare the Potatoes - Boil the peeled and cubed potatoes in salted water until fork-tender, about 15-20 minutes. Drain and return to pot.
Step 2: Mash the Potatoes - Add butter to the drained potatoes and mash until smooth. Incorporate egg and cornstarch, mixing well. Set aside to cool.
Step 3: Cook the Filling - In a frying pan, heat olive oil over medium heat. Add chopped onion and sauté until translucent. Stir in minced garlic and cook for another minute. Add ground beef, breaking it up until browned.
Step 4: Season the Beef - Once browned, stir in sofrito, tomato sauce, adobo seasoning, and sazón seasoning. Cook until well combined and heated through. Remove from heat and let cool.
Step 5: Shape the Relleno De Papa - Take a small portion of potato mixture, flatten it, add beef filling in the center, mold the potato around it to form a ball. Repeat until all mixture is used.
Step 6: Coat the Relleno - Roll each ball in cornstarch and breadcrumbs for extra crunch if desired.
Step 7: Fry the Relleno De Papa - Heat oil in a frying pan over medium heat. Add potato balls, frying until golden brown on all sides, about 3-5 minutes. Remove and drain on paper towels.
Step 8: Serve and Enjoy! - Serve hot with your preferred dipping sauce. Enjoy!