Preheat oven to 400℉. Place 10-inch cast iron skillet in the oven to heat up.
In a large bowl, whisk together cornmeal, flour, sugar, baking powder, salt, and cinnamon.
In another bowl, mix mashed yams, eggs, milk, melted butter, vanilla until smooth.
Pour wet ingredients into dry ingredients and stir just until combined. Do not overmix.
Carefully remove the hot skillet from the oven and lightly grease with butter. Pour batter into the hot skillet, it should sizzle slightly.
Bake for 20-25 minutes, or until the top is golden and a toothpick inserted in the center comes out clean.
Let cool for 10 minutes before slicing. Serve warm.