Methods:
Preheat oven to 350 F and grease a square baking pan.
Whisk eggs, sugar, oil, milk, and vanilla until smooth.
Stir in flour and baking powder just until combined.
Pour batter into the pan.
Sprinkle cinnamon evenly and swirl gently with a knife.
Bake 35 to 40 minutes until set.
Cool slightly before slicing.
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