Prep the pan. Heat the oven to 350°F (175°C). Grease a 9-inch springform pan or a 9–10 inch cake pan. Line the bottom with parchment for easy release.

Mix dry ingredients. In a bowl, whisk 2 cups (250 g) all-purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking soda, and 1/2 teaspoon kosher salt. Set aside.

Combine the sugars and butter. In a large bowl, beat 6 tablespoons softened unsalted butter with 1/2 cup (100 g) granulated sugar and 3/4 cup (150 g) packed light brown sugar until creamy, about 2 minutes. Stir in 6 tablespoons melted and cooled unsalted butter until smooth.

Add eggs and vanilla. Beat in 1 large egg, 1 egg yolk, and 2 teaspoons vanilla extract until glossy and well combined.

Bring it together. Add the dry ingredients to the wet and mix on low just until no dry spots remain. The dough will be thick.

Fold in chocolate. Stir in 1 1/2 cups (270–300 g) semisweet chocolate chips or chunks. Reserve a small handful to press on top for a pretty finish.

Press and top. Press the dough evenly into the prepared pan. Scatter the reserved chocolate on top. If you like, add a pinch of flaky salt.

Bake. Bake 20–26 minutes, depending on pan size and desired doneness. The edges should be set and lightly browned, and the center should look slightly underbaked but not raw. A toothpick should come out with moist crumbs, not wet batter.

Cool briefly. Let the cookie cake cool in the pan for 20–30 minutes. This helps it set for clean slices while keeping the middle soft.

Decorate and serve. Run a knife around the edge, remove from the pan, and transfer to a board. Pipe frosting around the border or add sprinkles. Slice into wedges and serve warm or at room temperature.