Preheat oven to 180c.
Grease and line a 20cm square pan (8-inch) with baking paper and set aside.
Place the salted butter in a large bowl and microwave to melt.
110 g salted butter
Add white sugar, light brown sugar and vanilla extract to the melted butter and mix to incorporate.
100 g white (granulated) sugar, 65 g light brown sugar, 1 teaspoon vanilla extract
Add the egg and mix well.
1 large egg
Add plain flour and cream of tartar and mix until combined.
140 g plain (all-purpose) flour, ¼ teaspoon cream of tartar
Pour batter into the greased and lined baking tin, and smooth the top.
Combine extra white sugar and cinnamon in a small bowl, then sprinkle the cinnamon-sugar mixture evenly over the top of the batter and bake for 20 to 22 minutes.
1½ tablespoon white (granulated) sugar, 1 teaspoon ground cinnamon