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 it super easy to lift the whole slab out later. If you don't have parchment, you can lightly grease the pan instead.

In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set this mixture aside.

In a large mixing bowl, combine the melted and slightly cooled butter with both the packed brown sugar and white sugar. Mix thoroughly until they are well combined and smooth. This creates the sugary foundation for our bars.

Add the eggs to the sugar-butter mixture, one at a time, mixing well after each addition. Stir in the vanilla extract until it's fully incorporated. The mixture should look creamy and light.

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 bars.

Fold in the chocolate chips. If you’re using optional toffee bits, don't mix them into the dough yet; save them for the top.

Press the dough evenly into your prepared 9x13 inch baking pan. Use the back of a spoon or your hands to get an even layer. If using toffee bits, sprinkle them evenly over the top of the pressed dough now.

Bake for 25-35 minutes, or until the edges are golden brown and the center looks set but still slightly soft. Remember, the center will firm up as it cools, so don't overbake them!

Remove the pan from the oven and place it on a wire rack to cool completely. This is the hardest part, but it’s crucial! If you cut them while warm, they'll be very gooey and fall apart. Let them cool fully to room temperature before lifting them out of the pan using the parchment paper overhang and cutting into squares.