Preheat your oven to 180°C (fan 160°C). Line a 20cm x 20cm square baking tin with baking parchment, leaving some overhang on two sides so you can lift the bars out later. You should hear the parchment crinkle as you press it into the corners.

Whisk the melted butter and sugars together in a large mixing bowl until the mixture looks glossy and smooth, like wet sand. It should smell deeply buttery and sweet. Give it a good minute of whisking so the sugars begin to dissolve.

Add the eggs one at a time, whisking vigorously after each addition. The batter will turn pale and thicken noticeably—you will see it pull away from the sides of the bowl in ribbons when you lift the whisk.

Stir in the vanilla extract until evenly combined. The mixture should now be a uniform, creamy colour with a fragrant, warm aroma.

Sift the flour, cream of tartar, bicarbonate of soda, and salt directly into the wet ingredients. Fold gently with a spatula until just combined—stop the moment you cannot see any white streaks. Over-mixing will make the bars tough, so look for a soft, shaggy dough that holds together when pressed.

Stir together the topping ingredients in a small bowl: 30g granulated sugar and 2 teaspoons ground cinnamon. The cinnamon should smell aromatic and spicy as you mix.

Transfer the dough to your prepared tin and press it evenly into the corners using a spatula or your fingertips. The dough will be quite thick and slightly sticky, so dampen your fingers slightly if needed. Sprinkle the cinnamon-sugar topping evenly over the surface, covering every bit of dough.

Bake for 20–22 minutes. The edges will turn a deep golden brown and pull away slightly from the parchment, while the centre will look puffed and set but still feel soft to a light touch. A skewer inserted into the middle should come out with a few moist crumbs attached—not wet batter.

Cool the tin on a wire rack for 15 minutes. Use the parchment overhang to lift the whole slab out and transfer to a cutting board. The bars will firm up as they cool, so resist the urge to slice them while hot. You will hear a slight crackle as the sugar crust settles.

Slice into 16 equal squares using a sharp, serrated knife. Wipe the blade clean between cuts for neat edges. The bars should feel dense and springy when you press the tops.