Preheat oven to 350°
Butter a 9" x 13" baking pan and dust with cocoa powder. In medium to large mixing bowl, melt butter. Using hand or stand mixer, blend in sugar for about 1 minute and then vanilla.
1 cup unsalted butter, 2 cups sugar, 2 teaspoon vanilla
Add eggs one at a time, mixing well after each addition.
4 eggs
Add cocoa to butter and egg mixture and mix on low until incorporated.
¾ cup unsweetened cocoa
Next, add flour, baking powder and salt to the top of the ingredients already in the bowl. I like to blend them loosely with a fork before turning on the mixer to bring them all together. Then, turn on mixer and mix for about a minute until everything looks blended.
1 cup flour, ½ teaspoon baking powder, ¼ teaspoon salt
Add both kinds of chocolate chips and mix on low.
1 cup semi-sweet chocolate chips, ½ cup white chocolate chips
Pour batter into prepared pan.
Bake for 30-35 minutes checking when you first smell the brownies likely around 25 minutes. Bake until cake tester or toothpick comes clean.
Cool completely and cut into squares.