Preheat oven to 375 degrees. Adjust oven rack to middle position. Line an 8-inch square baking pan with parchment paper or tinfoil. Spray with non-stick cooking spray. Set aside.

In a small bowl, whisk together flour, baking powder and salt. Roughly chop 3 ounces of the chocolate and place in a microwave-safe bowl. Microwave at 50 percent power in 20 seconds intervals until melted and smooth, stirring often. Chop remainder of chocolate into 1/2-inch pieces.

Using a stand mixer fitted with a paddle attachment, beat butter and 1/2 cup sugar at medium speed until light and fluffy, about 3 minutes. Add egg and vanilla and beat until combined. Add the melted chocolate and mix until combined, about 30 seconds. Reduce speed to low and slowly add the flour mixture. until combined, about 1 minute. Stir in the chopped chocolate.

Transfer batter to the prepared pan, spreading evenly. Toss the raspberries in the remaining 2 teaspoons sugar to coat. Top batter with raspberries. Bake for 25-35 minutes, until edges are puffy and golden brown, and a toothpick inserted in center comes out with a few moist crumbs.

Let bars cool on wire rack for 2 hours. Remove bars from pan and sprinkle with confectioners' sugar or drizzle with some additional melted white chocolate. Cut into 16 bars.