Preheat the oven to 375°F. Place paper liners in the wells of a muffin pan and measure all ingredients before beginning.
In a large mixing bowl, whisk the oat flour, brown sugar, pumpkin pie spice, baking powder, baking soda, and salt for approximately 30 seconds. Thorough whisking helps distribute the leavening agents evenly throughout the mixture.
In a separate medium bowl, whisk the pumpkin purée, room-temperature egg, milk, neutral oil, and vanilla until smooth and evenly blended.
Add the ground flaxseed, brewer’s yeast, and rolled oats to the pumpkin mixture. Stir gently until all three ingredients are incorporated.
Pour the wet mixture into the bowl containing the dry ingredients. Using a rubber spatula, fold the mixtures together only until no visible streaks of dry flour remain. The finished batter may retain a slightly uneven or lumpy appearance.
Divide the batter among the prepared muffin cups, filling each liner approximately three-quarters full.
Bake for 18 minutes. The muffins are ready when a toothpick inserted into the center comes out clean or carries only a few moist crumbs.
Allow the muffins to rest in the pan for about 5 minutes. Transfer them to a wire cooling rack and leave them to cool completely.