Preheat oven to 375°F (191°C). Line a light-colored 12×17-inch baking sheet (half sheet pan)* with parchment paper. Set aside.

Whisk the flour, baking soda, and salt together in a large bowl. Set aside.

Whisk the melted butter, granulated sugar, brown sugar, egg yolk, and vanilla extract together until combined. Pour into dry ingredients and, using a silicone spatula, mix everything together until completely combined. Fold in the mini chocolate chips. Dough will be very soft and look greasy.

Press the dough into the prepared pan, to an even thickness—covering the entire parchment paper-lined baking sheet. I like to use my hands to press it down at first, and then I switch to an offset spatula to really get the dough to all edges of the pan. It will take a few minutes, but I promise eventually you will get it pressed thin enough to cover the entire pan.

Bake for 18–20 minutes, or until a deep golden brown on top. Keep an eye on it after 15 minutes. If you are using a dark baking sheet, the bake time will be shorter than if you are using a light silver baking sheet, because a dark metal pan absorbs more heat. Again, keep an eye on it.

Remove from the oven and sprinkle the chopped chocolate all over the surface of the cookie. Return to the oven and bake for another 1–2 minutes, to melt the chocolate.

Place the pan on a cooling rack, and with an offset spatula (or the back of a spoon), spread the chocolate in an even layer over the cookie layer. Top with flaky sea salt or sprinkles, if using. Allow to cool for 20 minutes, then transfer the pan, still on the cooling rack, to the refrigerator, and chill for at least 30 minutes, until the chocolate is set.

Break or cut the cookie bark into bite-size pieces and enjoy!

Store leftover cookie bark at room temperature in an airtight container (with parchment paper between layers) for up to 5 days. We noticed it starts to taste a bit stale around day 4 or 5. But really, is it even possible to have any of this chocolate chip cookie bark left for that long? I doubt it!