Preheat oven to 350°F. Whisk masa harina, baking powder, and 1 tsp. garlic salt in a large bowl. Add hot water and olive oil. Knead until masa dough forms and no longer sticks to hands, 8 to 10 minutes.
Coat a 12-cup muffin pan with cooking spray; set aside. Divide masa dough evenly into 12 pieces. Roll each piece into a 1 3/4-in. ball. Press 1 masa dough ball evenly into bottom and up sides of each greased muffin cup. Bake in preheated oven until masa is cooked through, 28 to 30 minutes.
Meanwhile, heat vegetable oil in a large skillet over medium. Add onion and garlic; cook, stirring often, until onion softens slightly, about 3 minutes. Add tomato, and cook, stirring often, until tomato begins to soften, about 2 minutes. Add chicken, tomato sauce, oregano, cumin, black pepper, and remaining 1/4 tsp. garlic salt. Cook over medium heat, stirring often, until tomato sauce reduces and clings to chicken, about 8 minutes. Remove from heat.
Remove masa cups from pan. Spoon 3 Tbs. chicken filling evenly into each cup and top with cheddar and cilantro.