Preheat the oven to 350°F and line with parchment paper or grease a 9-inch square-baking pan or 9-inch cast-iron skillet.
On low heat, melt the butter. Remove from the heat and allow to cool, about 10 minutes.
Once cool, add to the melted butter the brown sugar, eggs, and vanilla extract. In a separate bowl, whisk together the flour, baking powder, salt, and orange zest until well combined.
Add the dry ingredients to the liquid ingredients and stir at least 20 times until a thick, light batter is formed. The batter should not be runny or soupy! The stirring is an important step because if it the batter isn't mixed enough, it won't be filled with enough air, which can lead to it not setting. Feel free to use a mixer if you prefer not to stir by hand.
Spread the batter into the baking pan. Stir together the sugar and cinnamon then spoon evenly over the batter. Bake uncovered for 20-25 minutes or until the edges are set. (Be sure to not overcook as the churro brownies will harden as they cool.) Lightly sprinkle with the flaked sea salt, then allow to cool for at least 20 minutes before slicing and serving.