In the bowl of a stand mixer (or a regular bowl if you don't have a stand mixer), cream the butter and sugars together until smooth. Add eggs and vanilla extract and beat well. Slowly add in the flour, cocoa, baking soda, and salt. Fold in 1 cup of white chocolate chips. Spread in a well greased 15x10x1 inch pan. Sprinkle the remaining white chips over the top of the batter. Bake at 375 degrees for 18-23 minutes. If you don't have a 15x10 pan, you can use a 9x13 pan to make the cookies instead. They will be thicker and will need longer to cook (closer to 30-40 minutes). Test with toothpick near the center, if it comes out clean, then they are ready (be sure to do this step with cookie bars - it's important so you don't have doughy/soggy cookie bars in the middle!) Let the cookie bars cool in the pan before cutting.