In a large bowl, whisk together the flour and brown sugar to break up any clumps. Add the egg, melted (but not hot) coconut oil, vanilla, pumpkin purée, and potato flake starter. Mix with a hand mixer or wooden spoon until the batter is smooth and thick. Scrape down the sides of the bowl as needed.

If the mixture seems too thick, add a small splash of milk or water to loosen it slightly.

Cover the bowl with plastic wrap or a loose-fitting lid and let it rest in a warm spot for 8–12 hours, or overnight. The batter should puff slightly and may have small bubbles on top—signs that the fermentation worked.

When ready to bake, preheat your oven to 350°F (175°C). Grease an 8x8-inch baking dish with butter, coconut oil, or nonstick spray, or line it with parchment paper for easy removal.

Stir the baking soda, salt, cinnamon, and pumpkin pie spice into the fermented batter. Mix gently until just combined—overmixing can make the bars dense.

Pour the batter into the prepared dish, smoothing the top evenly. Bake for 20–25 minutes, or until the center springs back lightly when pressed. A toothpick should come out mostly clean with a few moist crumbs.

Set the dish on a wire rack and let the bars cool completely before glazing.

In a small bowl, whisk together the melted butter, powdered sugar, espresso (or milk), maple extract, and pumpkin pie spice until smooth. Adjust thickness by adding more sugar or a splash more liquid as needed.

Spread or drizzle the glaze evenly over the cooled bars. Let it set for 15–20 minutes before slicing. For an extra touch, sprinkle a few chocolate chips or drizzle with maple syrup before serving.