Preheat oven to 350°F. Butter an 8- or 9-inch cake pan; line bottom of pan with parchment paper. In a medium bowl, whisk together flour, cocoa powder, baking soda and salt. In a large bowl, stir sugar and melted butter until well combined. Stir in egg. Add flour mixture, stirring just until moistened. Stir in stout, malt vinegar and vanilla until smooth. Pour into prepared pan. Bake 30 minutes or until a toothpick inserted into center comes out clean. Cool in pan 10 minutes. Run a knife around edge to loosen. Invert onto a wire rack and remove parchment; cool completely. In a blender, combine raspberries (reserving a few for garnish), sugar and balsamic vinegar; puree. Strain through a fine-mesh strainer into a small bowl. Serve cake garnished with whipped cream, if desired, and drizzled with coulis.