Preheat oven to 350 degrees and thoroughly grease a 9×13 pan.
Mix the melted butter and sugars together in a large bowl until smooth. While continuing to mix, add the eggs and vanilla.
Add the flour, baking powder, cream of tartar, and salt to the bowl of wet ingredients and stir until combined.
Transfer batter to the prepared baking pan and press into an even layer.
In a small bowl, combine sugar and cinnamon for the topping, then sprinkle over the batter.
Bake for 20-25 minutes or until the top is just turning golden.
Allow to cool for 30 minutes before slicing and serving.
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