Prep the pan: Heat the oven to 350°F (175°C). Grease a 9-inch or 10-inch round cake pan or springform pan. Line the bottom with parchment for easy release. For thicker, gooier slices, use a 9-inch; for crisper edges and slightly thinner slices, use a 10-inch.

Melt the butter: Melt the butter and let it cool for 5–7 minutes so it’s warm, not hot. Warm butter blends easily without scrambling the eggs.

Mix wet ingredients: In a large bowl, whisk the brown sugar and granulated sugar with the melted butter until glossy and smooth, about 45 seconds. Whisk in the eggs one at a time, then the vanilla and milk.

Combine dry ingredients: In a separate bowl, whisk the flour, cornstarch, baking soda, and salt. Cornstarch adds tenderness and helps the center stay soft.

Make the dough: Add the dry ingredients to the wet ingredients and fold with a spatula until just combined. Don’t overmix. Stir in the chocolate chips (and nuts, if using), reserving a small handful for topping.

Pan and top: Press the dough evenly into the prepared pan. Scatter the reserved chips over the top for a bakery-style look. If you like, sprinkle a pinch of flaky sea salt.

Bake: Bake for 20–26 minutes, depending on pan size and how gooey you like it. The edges should be set and golden, and the center should look slightly underbaked but not wet. A toothpick inserted near the center should come out with a few moist crumbs.

Cool and release: Set the pan on a rack and cool for 20–30 minutes. Run a thin knife around the edges, then lift out using the parchment or release the springform ring. The cookie cake continues to set as it cools, so don’t rush it.

Finish and serve: If decorating, pipe buttercream around the edges once cool to the touch. Slice into wedges. For a warm, gooey experience, serve slightly warm with ice cream.