Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper or foil, leaving an overhang on the sides. This makes lifting the bars out much easier. You can also lightly grease the pan if you don't have paper.
In a medium bowl, whisk together the all-purpose flour, baking soda, and kosher salt until they are well combined. Set this mixture aside.
In a large mixing bowl, combine the melted and slightly cooled butter with both the brown sugar and white sugar. Mix thoroughly until the sugars are completely incorporated and the mixture is smooth.
Add the eggs to the butter and sugar mixture, one at a time, mixing well after each addition. Stir in the vanilla extract until it's fully combined.
Gradually add the dry flour mixture to the wet ingredients. Mix on low speed, or by hand, until just combined. Be careful not to overmix the dough, as this can lead to tough cookie bars.
Stir in the chocolate chips until they are evenly distributed throughout the dough. If you're using optional toffee bits, stir those in now as well, or save some to sprinkle on top.
Press the cookie dough evenly into your prepared 9x13 inch baking pan. Make sure the surface is relatively flat for even baking. If you saved any chocolate chips or toffee bits, sprinkle them over the top now.
Bake for 25 to 35 minutes. Start checking at 25 minutes. The edges should be golden brown, and the center will look set but still soft. It will seem a little underbaked, but that's what gives them their amazing chewy texture after cooling.
Remove the pan from the oven and let the bars cool completely on a wire rack before attempting to cut them. This is the hardest part, but it's crucial! Cutting them too soon will result in a gooey, crumbly mess. Patience is key for perfect squares.
Once fully cooled, use the parchment paper overhang to lift the entire slab of cookie bars out of the pan. Place them on a cutting board and slice into squares.