Preheat oven to 350° and grease a 13 x 9 baking dish. Set aside.
In a large mixing bowl, whisk flour, sugar, cocoa powder, baking powder and salt together.
Pour milk, oil and vanilla into flour mixture, stir until combined then stir in walnuts.
Add batter mixture to prepared baking dish.
In a medium mixing bowl, whisk brown sugar and cocoa powder together.
Slowly add boiling water into mixture, stir, then carefully pour pudding mixture over brownie mixture.
Bake for 40 minutes then remove from oven and place on a wire rack to cool. After about 15 minutes, add chocolate chips, white chocolate chips and walnuts on top. Let cool for 30 more minutes.
Spoon into dessert bowls and spoon additional pudding over the top.
Serve warm with a scoop of vanilla ice cream.
Recipe mostly adapted from BHG