Preheat the oven to 350 degrees.

Cream together the butter and sugars with an electric mixer until light and fluffy, 2-3 mins.

Add the eggs one at a time, beating after each addition. Mix in the vanilla extract.

Sift the flour, cream of tartar, baking soda and salt together and mix it into the butter mixture on low speed until combined. The batter will be very thick.

Grease a 9x13 inch pan and line it with parchment paper if desired.

Spread the batter into the pan and smooth. You can use cooking spray to grease a spatula to help you spread it into an even layer. Set aside.

Cinnamon Sugar Topping: Combine all the ingredients in a bowl and stir to combine.

Drop on top of the batter in spoonfuls and smooth over with a knife.

Bake for about 20 mins or until the topping is just set.

To serve, slice into 16 squares.