Grind the corn in a food processor until the kernels break apart and the mixture is smooth. Transfer the ground corn to a large bowl and add the cornmeal, sugar, flour, baking powder, salt, the ricotta cheese, and Manchego cheese.
Using a wooden spoon, mix the ingredients to combine. Add the 3 tablespoons butter and the milk and to stir until the mixture comes together. Do not overmix. In the beginning, your mixture will look like a very loose pancake batter. Don’t fret, the cornmeal will take a few minutes to absorb the liquid and achieve a better consistency. Allow the dough to rest for 10 to 15 minutes.
Meanwhile, prepare the raw tomato and avocado salad: In a medium bowl, combine the tomatoes and avocados. Add the lemon juice and olive oil and toss to combine. Season with salt and pepper to taste.
Line a baking sheet with parchment paper and set aside.
Preheat a large cast-iron skillet or griddle over medium-highheat. Melt the 2 tablespoons butter until bubbles form. Measure . cup of the batter and pour into the pan — pancake style. You can fry the arepas 2 at a time or more, depending on the size of your skillet. Do not crowd the skillet. Cook the arepas until golden brown, 2 to 3 minutes per side. You may need to turn down the heat as you go so that the pan doesn’t get too hot from batch to batch. Place the finished arepas on the baking sheet and keep warm in the oven. Repeat with the remaining batter.
To serve, plate each arepa and top with a spoonful of the tomato and avocado salad, the crumbled quesito, and cilantro leaves.