Preheat the oven to 325 degrees Fahrenheit. Grease a 9-inch pie pan.
Combine the flour, baking soda, and kosher salt in a bowl. Set aside.
In the bowl of a stand mixer (or with an electric mixer), cream together the butter, granulated sugar, and brown sugar with paddle attachment for 2 to 3 minutes until fluffy. Add the vanilla, egg, milk, and orange zest and beat for 1 minute.
With the mixer on low, add the flour mixture until just combined. Remove the bowl from the mixer and fold in 1/2 cup of the chocolate chips. Use a spatula to press down the dough into the pie pan. Top with the additional 1/4 cup chocolate chips.
Bake 25 to 27 minutes, until the edges are starting to brown and the center has fully firmed up. Cool several hours at room temperature before serving (the cookie cake is much too gooey to cut when it’s first baked, so the longer you can cool it the better!). Store at room temperature covered for up to 3 days or refrigerated for 1 week (bring to room temperature before serving.)