Preheat the oven to 325ºF. Line an 8x8 baking pan with foil and spray with cooking spray.
Place a heatproof bowl over a pot of simmering water. Put the butter and chopped chocolates in the bowl and stir occasionally until they are just melted. Remove the bowl from the pan of water.
Add the sugar to the melted mixture and whisk until incorporated. One by one, whisk in the eggs. Add the vanilla and whisk vigorously. Stir in the flour, followed by the walnuts.
Scrape the batter into the prepared pan and smooth the top.
Bake 30-33 minutes or until a toothpick inserted in the center comes out clean.
Let cool on a wire rack. When cool, lift the brownies out of the pan using the foil and cut into squares.