Preheat oven to 350 degrees Fahrenheit.

In a medium bowl, combine flour and brown sugar.

Cut in cold butter until the mixture resembles coarse crumbs.

Stir in 1/2 cup of chocolate chips.

Press the mixture firmly onto the bottom of a 13x9x2 inch baking pan.

Bake for 15 minutes.

While the crust is baking, in a large bowl, combine sweetened condensed milk, egg, and vanilla extract.

Stir in the remaining 1 1/2 cups of chocolate chips and chopped nuts.

Spread the chocolate chip mixture evenly over the baked crust.

Bake for an additional 25 minutes, or until golden brown.

Cool completely before cutting into bars.