Preheat oven to 350 degrees F. Line an 8x8x2-inch baking pan with parchment paper leaving the sides overhanging or butter and flour.
In a large bowl whisk the flour, baking powder and salt together.
In a microwave safe bowl melt 1/2 cup semisweet chocolate chips using the chocolate setting of your microwave in 20 second increments, stirring in between until smooth and melted. Whisk in the sugar, cocoa powder, vanilla and yogurt.
Fold the wet ingredients into the dry ingredients. Mix in 1/4 cup semisweet chocolate chips and 1/4 cup white chocolate chips and walnuts.
Pour the batter into the pan and spread evenly. Sprinkle the remaining 1/4 cup semisweet and 1/4 cup white chocolate chips over top. Bake 28 - 32 minutes or until a toothpick in center comes out with a few moist crumbs but not wet batter.
Cool in pan on wire racks. Lift using the parchment to a cutting board and cut into squares with a sharp chef's knife or paring knife, wiping clean after each cut.
Store tightly covered in a cool, dry place.