1. Preheat oven to 350°F (175°C) and line an 8×8 inch baking pan with parchment paper.

2. In a heatproof bowl over simmering water, melt the butter and chopped chocolate, stirring until smooth, then let cool slightly.

3. In a large bowl, whisk together granulated sugar, brown sugar, eggs, and vanilla until smooth and slightly thickened.

4. Pour the melted chocolate mixture into the sugar and egg mixture and whisk until glossy and uniform.

5. In a separate bowl, sift together flour, cocoa powder, and salt.

6. Gently fold the dry ingredients into the chocolate mixture until just combined.

7. Fold in raspberries and optional chocolate chips, being careful not to overmix.

8. Pour batter into the prepared pan and smooth the top.

9. Bake for 30 to 35 minutes until edges are set and center yields moist crumbs to a toothpick.

10. Cool in the pan on a rack for 20 minutes, then lift out using parchment overhang and cool completely before slicing.

11. Slice into 12 even squares and serve with optional powdered sugar dusting or fresh raspberries.