Preheat oven to 350°F and grease a donut pan.
Add the spinach, banana, egg, Greek yogurt, maple syrup (or honey), and vanilla to a blender and blend until completely smooth.
Pour into a bowl, then add the oat flour, almond flour, cocoa powder, baking powder, baking soda, and salt. Stir until just combined.
Spoon or pipe the batter into the donut pan, filling each about 3/4 full. Top with chocolate chips if using.
Bake for 15–18 minutes, until the centers are set and a toothpick comes out with moist crumbs.
Let cool for about 10 minutes before removing from the pan. They will continue to firm up as they cool.