Preheat your oven to 350°F and line a 9×9-inch baking pan with aluminum foil, leaving a 2-inch overhang on each side.
Grease or spray the foil to make sure the squares don't stick.
In a medium bowl, combine the flour, ground cinnamon, baking powder, and baking soda.
Set aside as you'll add these dry ingredients later.
In a large saucepan, melt the butter over low heat.
Once melted, remove it from the heat and mix in the sugar until fully dissolved.
Add the eggs to the butter and sugar mixture, then stir in the vanilla extract and salt.
Make sure everything is mixed well.
Gradually fold the dry ingredients into the wet mixture until just combined.
Avoid overmixing!
Pour the batter into your prepared pan and smooth it out evenly.