Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs and vanilla.

In another bowl, whisk together flour, cocoa, baking powder, and salt; gradually add to creamed mixture and mix well.

Stir in chocolate chips and white baking chips.

Divide dough in half. On the prepared baking sheet, shape each portion into a 12×2-in. rectangle, spacing them 4 inches apart.

Bake for 25-30 minutes or until firm and tops are cracked. Cool on pans for 10 minutes.

Transfer to a cutting board; cut diagonally with a serrated knife into 3/4-in. slices. Place slices cut side down on ungreased baking sheets.