Preheat oven to 350°F (175°C). Grease a 9x13 inch baking pan.

In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.

Beat in the eggs and vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Stir in the chocolate chips.

Press the dough evenly into the prepared baking pan.

Bake for 25-30 minutes, or until golden brown and the edges are set.

Let cool completely before cutting into squares.

Alternative Instructions:

Begin by preheating your oven to 350°F (175°C). Prepare a 9x13 inch baking pan by greasing it lightly.

In a large mixing bowl, combine the softened butter, granulated sugar, and packed brown sugar. Use an electric mixer or a whisk to cream these ingredients together until the mixture is light and fluffy.

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt to ensure they are evenly distributed.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined. Be careful not to overmix.

Fold in the semi-sweet chocolate chips and milk chocolate chips (if using) until they are evenly distributed throughout the dough.

Press the cookie dough evenly into the prepared baking pan, ensuring it reaches all corners.

Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and the center is set but still slightly soft.

Remove the pan from the oven and let the cookie bars cool completely in the pan before cutting them into squares. This will prevent them from crumbling.