Preheat oven to 350°F. Line or grease an 8x8 baking dish.
Whisk flour, baking powder, baking soda, cinnamon, and salt in a medium bowl.
In a large bowl, whisk melted butter, Greek yogurt, maple syrup, eggs, and vanilla until smooth.
Gently fold dry ingredients into wet ingredients until just combined.
Pour batter into pan. Sprinkle cinnamon sugar over the top and swirl gently with a knife, if using.
Bake 30–34 minutes, or until a toothpick comes out clean.
Cool completely before glazing (optional).