Preheat oven to 175°C (350°F). Line an 8-inch square tin with parchment paper.
Mix the topping sugars and set aside.
In a medium bowl, whisk the flour, bicarbonate of soda, cream of tartar, cinnamon, and salt.
In a stand mixer bowl, cream butter and sugars until fluffy. Add the egg and vanilla.
Add the dry ingredients to the wet and mix to form a dough.
Spread into the tin, smooth the top, and sprinkle with cinnamon sugar.
Bake for 23-26 minutes, or until golden around the edges.
Cool in the tin before slicing into squares.