Preheat and prepare: Preheat your oven to 180°C (160°C fan) and line a 23cm x 23cm square baking tin with parchment paper, leaving some overhang for easy lifting. The paper should sit flush against the corners.

Cream the butter and sugars: In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together with an electric mixer on medium speed until the mixture turns pale and fluffy—about 3 minutes. You will notice the colour lighten and the texture become airy, almost like a thick paste.

Add the eggs and vanilla: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. The batter will look glossy and smooth. Stir in the vanilla extract until you can smell its sweet, fragrant aroma.

Combine the dry ingredients: In a separate bowl, whisk together the plain flour, cream of tartar, bicarbonate of soda, and salt. This step ensures the leavening agents are evenly distributed—you will see the pale cream colour of the cream of tartar disperse through the flour.

Fold the dry into the wet: Gradually add the dry ingredients to the wet mixture, folding gently with a spatula or wooden spoon until just combined. Stop when you no longer see streaks of flour; the dough will be thick, soft, and slightly sticky to the touch.

Prepare the cinnamon-sugar: In a small, shallow bowl, whisk together the 60g of granulated sugar and the ground cinnamon. The deep, earthy scent of cinnamon should immediately fill your workspace.

Assemble the bars: Press half of the dough evenly into the prepared tin. It will be sticky, so lightly dampen your fingertips or use a piece of parchment paper to press it down into an even layer. Sprinkle half the cinnamon-sugar mixture evenly over the dough. Dot spoonfuls of the remaining dough over the top, then gently press it out to cover the layer. Finally, sprinkle the remaining cinnamon-sugar mixture over the top.

Bake to golden perfection: Bake for 22-25 minutes. At the 20-minute mark, the edges will look set and golden brown, while the centre will still appear slightly soft and puffy. The kitchen will smell of warm butter and cinnamon. A skewer inserted into the centre should come out with a few moist crumbs, not wet batter.

Cool completely: Allow the bars to cool in the tin on a wire rack for at least 30 minutes. This rest is crucial—the bars will continue to set as they cool. Once cool, use the parchment overhang to lift the block out onto a cutting board. Slice into 16 squares with a sharp knife, wiping the blade clean between cuts for neat edges.