Place a rack in the center of the oven and preheat to 350° degrees. Grease or line a 12-cup muffin tin with liners.
Melt the butter in a heavy saucepan over medium heat. Butter will begin to foam and crackle as it melts. When the crackling subsides, the butter will begin to brown. Swirl the pan as the butter cooks. When the butter browns and begins to smell nutty, remove the pan from the flame and transfer the butter to a small bowl. Taking the butter out of the hot saucepan will stop the butter from overcooking and burning. Set it aside to cool.
In a large bowl, whisk together the oat flour, sugar, baking powder, baking soda, salt, cinnamon, and cardamom.
In a medium bowl, whisk together eggs and yogurt, then whisk in the mashed bananas and maple syrup. When butter has cooled, whisk in the browned butter.
Add the wet ingredients all at once to the dry ingredients. Fold together, making sure to scrape the bottom of the bowl to reveal any hidden pockets of flour. Fold in the chocolate chips if using.
Preheat the oven to 350℉. Divide the batter equally between the muffin cups in the prepared pan. Rest the batter for up to 20 minutes, then bake for 20-25 minutes, or until a skewer inserted into the center of a muffin comes out clean. Remove from the oven and allow to rest in the pan for 15 minutes, before inverting onto a wire rack to cool completely.
Muffins may be stored in an air tight container in the fridge for 5 days.
Notes
Play with the spices - Try a blend of cinnamon and cardamom as written in the recipe, or go your own way. I love ginger in cozy baked treats, or if you really love a kick of ginger, add 1 tablespoon of peeled and grated fresh ginger.
Use a gluten-free flour blend - If you don't do oat flour, substitute your favorite gluten-free flour blend (I love my homemade gluten free flour blend in all of my recipes!). Use 2 ½ cups of blend for the oat flour.
Other nuts - Add chopped almonds or unsalted pistachio nuts instead of the walnuts.
Make dairy-free - Use melted coconut oil or olive oil instead of the melted butter, and use non-dairy yogurt in place of the yogurt.
Make refined sugar-free - Substitute coconut sugar or maple syrup for the brown sugar.