Preheat oven to 350°F (175°C). Use a 9-inch cast iron skillet or 8-inch square baking pan. Grease thoroughly with butter or cooking spray
In a large bowl, whisk together:
1 cup masa harina
1/2 cup all-purpose flour
1 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
In a separate bowl, combine:
1 (14-oz) can sweetened condensed milk
2 large eggs
1/4 cup unsalted butter, melted
1/4 cup vegetable oil
1 teaspoon vanilla extract
Optional: 1/4 cup milk (to adjust consistency if needed)
Pour wet ingredients into dry ingredients and gently fold until just combined. If batter seems too thick, add milk 1 tablespoon at a time. Batter should be thick but pour-able
Pour batter into prepared pan. Smooth top with a spatula. Bake 25-30 minutes until top is golden brown and inserted toothpick comes out with a few moist crumbs.