Preheat the oven - 180C/350F
Grease and flour (or line with baking paper) a 13x9 pan and set aside
In a large bowl, combine the melted butter, sugar and milk.
Add in the eggs and stir until combined.
Add the zest and juice of 1 lemon and stir until combined.
In a separate bowl, combine flour, baking powder, cream of tartar and salt.
Stir into the butter mixture and stir until combined.
Pour the final mixture into the prepared pan.
In a small bowl, prepare the topping by combining the sugar and cinnamon.
Sprinkle over the batter in the pan.
Bake for 25-28 minutes.