Preheat your oven to 425°F and prepare an 8x8-inch baking pan by spraying it with flour-based cooking spray or lining it with parchment paper.

In a medium bowl, whisk together the flour, cornmeal, brown sugar, baking powder, salt, and pumpkin pie spice. Set aside.

In a large mixing bowl, whisk together the pumpkin puree, sour cream, melted butter, maple syrup, and oat milk.

Gradually stir the dry ingredients into the wet ingredients until just combined. Allow the batter to rest for 10 minutes, giving the cornmeal time to absorb some of the moisture.

Pour the batter into the prepared baking dish. Bake for about 20 minutes or until golden brown and a toothpick comes out clean from the center. Cool for 10 minutes before cutting into squares. Serve warm or at room temperature.