1. Preheat the oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving overhang on the long sides, and lightly grease the parchment.

2. In a large bowl whisk together the melted butter, brown sugar, and granulated sugar until smooth and glossy.

3. Add the eggs and vanilla extract and whisk just until combined and no streaks of egg remain.

4. In a separate bowl whisk the flour, baking soda, baking powder, and fine sea salt.

5. Stir the dry ingredients into the wet ingredients with a spatula until just combined, then fold in the 1 1/4 cups semisweet chocolate chips for the cookie dough.

6. Press about two thirds of the cookie dough evenly into the prepared pan, making sure to cover the bottom and reach into the corners.

7. In a medium saucepan over low heat combine the 1 1/2 cups semisweet chocolate chips for the filling, sweetened condensed milk, and 2 tablespoons butter for the filling. Stir constantly until the chocolate is melted and the mixture is smooth and glossy.

8. Remove the pan from the heat and stir in the 1 teaspoon vanilla extract for the filling and 1/4 teaspoon fine sea salt for the filling.

9. Pour the warm fudge filling over the cookie dough base and spread it into an even layer without disturbing the bottom crust too much.

10. Drop spoonfuls of the remaining cookie dough over the fudge layer, flattening them slightly so you have little islands of dough with some fudge peeking through.

11. Sprinkle the 1/4 cup semisweet chocolate chips for sprinkling on top over the surface and add a pinch of flaky sea salt if using.

12. Bake for 25 to 30 minutes, or until the edges are golden and the top looks set but the center still feels slightly soft when gently jiggled.

13. Set the pan on a wire rack and let the bars cool completely, at least 1 to 2 hours, so the fudge layer can firm up.

14. Use the parchment overhang to lift the slab out of the pan, transfer to a cutting board, and slice into bars or squares before serving.