Preheat and prepare the tin: Preheat your oven to 180°C (350°F) fan-forced. Line your 23cm square tin with parchment paper, leaving a 5cm overhang on two opposite sides—this will be your sling for lifting the bars out later. The paper should lie flat against the corners.

Mix the wet ingredients: In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until the mixture looks glossy and thick, about 1 minute. You should smell the rich, caramel-like aroma of the butter and sugars combining. Add the whole eggs, egg yolk, and vanilla extract, then whisk vigorously until the batter is smooth, pale yellow, and slightly aerated—about 2 minutes. The texture will feel like a thick, pourable custard.

Combine the dry ingredients: In a separate bowl, sift together the plain flour, cream of tartar, bicarbonate of soda, salt, and nutmeg. Sifting is important here to break up the cream of tartar, which tends to clump. The flour mixture should look light and uniform in colour.

Fold the dough together: Pour the dry ingredients into the wet mixture. Using a rubber spatula, fold gently until just combined—stop when you no longer see streaks of flour. The dough will be very thick and slightly sticky, like a soft cookie dough. Do not overmix, or the bars will turn out tough. If the dough seems too soft to press, let it rest for 5 minutes; the flour will hydrate and firm up.

Layer the cinnamon sugar: In a small bowl, stir together the 60g granulated sugar and 2 tablespoons cinnamon until evenly mixed and fragrant. Spread about a third of the dough into the prepared tin, pressing it into an even layer with your fingertips. The layer should be about 1cm thick. Sprinkle half of the cinnamon-sugar mixture evenly over this first layer—you will see the sugar crystals glistening against the dough. Drop the remaining dough in spoonfuls on top, then gently press it out to cover the cinnamon layer completely. It will seem uneven, but that is fine; the second layer of dough will bake into a beautiful marbled effect. Sprinkle the remaining cinnamon sugar on top, ensuring full coverage right to the edges.

Bake to perfection: Bake for 25–28 minutes. At 22 minutes, the bars will look pale and jiggly in the centre—do not pull them out yet. Wait until the edges are a deep golden brown and have pulled away slightly from the parchment, and the centre is just set with a slight crackle on the surface. The top should feel dry to the touch but still soft underneath. A toothpick inserted into the centre should come out with a few moist crumbs, not wet batter. The smell of cinnamon and toasted butter will fill your kitchen.

Cool and slice: Place the tin on a wire rack and let the bars cool completely in the tin for at least 20 minutes. This resting time is critical—the bars will continue to set as they cool. After cooling, lift the bars out using the parchment overhang and transfer to a cutting board. Using a sharp knife, trim the edges for a clean look (you can snack on those trimmings), then cut into 16 even squares. For the most precise cuts, wipe the knife clean between each slice. The bars should be thick, about 2.5cm tall, with a crackly cinnamon crust and a soft, chewy interior.