The mixture may look slightly grainy from the sugar, which is completely normal.

Step 4 – Add the Egg, Yolk, and Vanilla

Add the whole egg, extra egg yolk, and vanilla.

Whisk vigorously for approximately 1 minute.

The mixture should become thicker, slightly paler, and ribbon-like.

Step 5 – Add the Dry Ingredients

Add the flour, milk powder, cornstarch, instant coffee powder, baking powder, and salt.

Use a rubber spatula to fold everything together gently.

Stop while a few streaks of flour are still visible.

Step 6 – Add the Chocolate

Fold in the semisweet chocolate chunks.

Continue folding only until the chocolate is evenly distributed and the final streaks of flour disappear.

The dough should be thick and soft.

Step 7 – Press Into the Pan

Transfer the cookie dough to the prepared pan.

Press it evenly into all four corners using a spatula or the back of a spoon.

Try to create an even layer so the bars bake consistently.

Press several additional chunks of chocolate into the surface.

These will create those irresistible glossy chocolate pools after baking.

Step 8 – Bake

Bake at 350°F (175°C) for approximately 18–22 minutes.

Begin checking around 18 minutes.

The edges should look golden and set.

The middle should appear softer and slightly underbaked.

Insert a toothpick into an area that is mostly cookie dough rather than melted chocolate.

You want to see moist crumbs.

If the toothpick comes out completely clean, the bars may already be more baked than necessary for the gooey texture we're after.

Step 9 – Cool Completely

Place the baking pan on a wire rack and leave the bars inside the pan until completely cool.

As tempting as it is to cut into them immediately, cooling is part of the baking process here.

The center continues setting as the temperature gradually drops.

For clean, bakery-style squares, cool for 1–2 hours before cutting.

Step 10 – Slice and Finish

Lift the entire cookie slab from the pan using the parchment paper.

Use a sharp knife to cut it into:

• 9 generous bars, or
• 16 smaller squares.

For especially neat slices, wipe your knife clean after each cut.

Add a very light sprinkle of flaky sea salt if desired.

The Secret to Gooey Chocolate Chip Cookie Bars

There are two things working together here: fat and carryover cooking.

The extra egg yolk increases richness without adding too much water, helping the bars stay dense instead of cakey.

Then there is the bake time.

Because the pan remains extremely hot after leaving the oven, the center continues setting even though it initially appears slightly underdone.

If you wait until the center looks completely baked before removing the pan, that residual heat can push the bars past the ideal stage.

That is why the visual cue matters more than blindly following a timer.

Set edges + soft center + moist crumbs = exactly what you want.

The Secret to Deep Brown Butter Flavor

Do not stop browning your butter the moment it turns pale gold.

The flavor develops in the milk solids.

They need enough time to toast until they become amber-brown and smell intensely nutty.

That is when the flavor moves beyond regular melted butter and develops those complex roasted, caramelized notes.

Just remember that once those solids reach the correct color, things move quickly.

Pour the butter out of the hot saucepan immediately so it does not continue cooking and burn.