In a medium pot, combine 4 cups of whole milk with granulated sugar, lemon peel, orange peel, and a cinnamon stick. Place the pot over medium heat and bring to a gentle boil. Once boiling, remove from heat and let sit until it cools to room temperature for about 30 minutes.

While the milk cools, whisk together ½ cup of the remaining milk with cornstarch and flour in a bowl until completely smooth and lump-free.

Remove the lemon peel, orange peel, and cinnamon stick from the cooled milk, and transfer the infused milk to a larger pot. Gradually mix in the cornstarch and flour mixture while stirring to combine. Cook over medium-high heat, stirring constantly for about 6-7 minutes, until it thickens significantly.

Line a baking dish with parchment paper. Pour the thickened Leche Frita mixture into the dish, spreading it evenly. Cover with plastic wrap and refrigerate for at least 3 hours, or overnight, until firm.

Once set, lift it out using the edges of the parchment paper. Cut into squares or rectangles. Prepare an egg wash with medium eggs in one bowl and a shallow dish with flour next to it.

In a large skillet, heat sunflower oil over medium-high heat until it reaches approximately 350°F. Dip each piece into flour, then the egg wash. Fry in hot oil for 2-3 minutes per side, or until golden brown.

Remove fried pieces from the skillet using a slotted spoon and place them on paper towels to drain. Mix powdered or granulated sugar with ground cinnamon and coat each piece while still warm.