Preheat oven to 325 degrees. Butter and dust with cocoa powder a 9x13" pan. (You can also line the pan with parchment paper, leaving excess paper over the sides.) Set aside.

Place a heat-resistant (glass or stainless steel bowl) over a pan of simmering water. Place the butter in the bowl to begin melting. In a separate bowl, combine the sugar, cocoa powder and salt. When the butter is completely melted, gradually add the sugar mixture to the butter until fully combined. Remove from heat, and add the vanilla. Let mixture cool for about 10 minutes.

To the cooled mixture whisk in the eggs one at a time, making sure to beat the mixture well. Mix together the flour and baking powder, and gradually add this to the batter until completely combined. Fold in the white chocolate chips. Spread the batter evenly into the prepared pan. Bake at 325 for about 45 minutes or until toothpick inserted comes out with crumbs. Let brownie cool completely.

To the cooled brownie, pour the melted chocolate chips over the top and even spread. Add dolops of the melted white chocolate chips, and swirl together with a spatula. Let cool completely before cutting (place in the refrigerator to speed up the cooling/setting process.) Enjoy!