Preheat oven to 350°.
Mix flour, powdered sugar, and cocoa in a large bowl.
Cut in cold butter pieces with a pastry cutter into flour mixture until crumbly.
Press into a 13" x 9" pan and bake at 350° for 15 minutes.
Place pan on a wire rack to cool slightly.
In a medium saucepan, add sweetened condensed milk, vanilla, and 1 cup of chocolate chips.
Heat over low heat until melted and smooth.
Spread evenly over the shortbread crust and then sprinkle the remaining ½ cup chocolate chips over the top and bake at 350° for 20 minutes.
Let cool on a wire rack.