This is where texture gets built, so don't rush the creaming stages.

Beat the butter and sugars for a full 3 to 4 minutes. You want the mixture pale, fluffy, and visibly lighter in color. This isn't just mixing, it's aerating. Skipping this step means denser bars with less of that signature chew.

Add the eggs, yolk, and vanilla, then beat again for another 3 to 4 minutes. The mixture should look like whipped frosting at this point. That extra yolk adds richness and keeps the center soft even after cooling. If it looks curdled, keep beating. It'll come together.

Whisk the dry ingredients separately. Combine the flour, cream of tartar, baking soda, and salt in another bowl. This ensures the leavening is evenly distributed so you don't get pockets of flat or overly puffy dough.

Fold the dry mix into the wet ingredients gently. Use a spatula or your hand and stop as soon as the white streaks disappear. Overworking the dough here leads to tough, cakey bars instead of chewy ones. Treat it like you're folding in something fragile.

Press the dough into a parchment-lined pan. Use a 9x13 for thick bars or an 8x8 for extra-thick ones. Grab a small piece of parchment and use it to press the dough down evenly. Your hands will thank you, and the surface will stay smooth.

Blanket the top with the entire cinnamon-sugar mixture. Don't be shy. Use all of it and press it into the dough with your hand or spatula so it sticks. This is what creates that crackly, sugary crust you're after.

Bake at 350 degrees for 25 to 30 minutes. Look for puffy edges and a center that's set but not browned. If they look fully done in the oven, they're overdone. The bars will carry-over cook in the hot pan for at least 30 minutes after you pull them out. That resting time is what locks in the soft, chewy heart.

Patience during that cooling window is everything. Cut them too early and they'll fall apart. Wait it out and you'll get clean squares with perfect texture.