Preheat oven to 350 degrees Fahrenheit. Line a 9x9 inch square baking pan with parchment paper, leaving an overhang for easy removal.

In a mixing bowl, cream together 1 cup softened butter and 1/2 cup granulated sugar until light and fluffy, about 2 minutes. Stir in 2 cups all-purpose flour and 1/2 teaspoon salt until mixture resembles coarse sand.

Press the shortbread mixture firmly and evenly into the bottom of the prepared pan. Bake for 12 minutes, until edges are light golden and center looks slightly underbaked. Let cool for 5 minutes.

In a separate bowl, cream 1/2 cup softened butter with 1/4 cup packed brown sugar and 1/4 cup granulated sugar until fluffy, about 1 to 2 minutes. Beat in 1 large egg and 1 teaspoon vanilla extract until combined.

Whisk together 3/4 cup all-purpose flour, 1/2 teaspoon baking soda, and 1/4 teaspoon salt in another bowl. Fold the dry ingredients into the wet ingredients, then fold in 1 cup chocolate chips. Spread the cookie dough layer evenly over the warm shortbread base.

In a saucepan over low heat, melt 1/2 cup butter and 4 ounces chopped unsweetened chocolate together, stirring frequently until smooth. Remove from heat and let cool for 2 minutes.

Stir in 1 cup granulated sugar, then whisk in 2 large eggs and 1 teaspoon vanilla extract until glossy. In a small bowl, whisk together 1/2 cup all-purpose flour and 1/4 teaspoon salt, then gently fold into the chocolate mixture until just combined.

Pour the brownie batter over the cookie dough layer and spread evenly to the edges. Bake at 350 degrees Fahrenheit for 23 to 25 minutes, until the top is set but the center still feels slightly soft.

Remove from oven and cool completely on a wire rack for at least 1 hour. Use the parchment overhang to lift the brownies from the pan and cut into 16 equal squares.