Bake the brownies according to the package directions, adding the extra egg, substituting water for milk and using an 8-inch square baking pan. Cool completely.
Meanwhile, put the raspberries, 1/4 cup of super-fine sugar and the salt in a blender and puree until smooth. Push through a sieve with a rubber spatula; discard the seeds. (The raspberry coulis can be made 3 days in advance and stored, covered, in the refrigerator.)
Whip the heavy cream with the remaining 1/2 cup super-fine sugar until soft peaks form. Cut out four 3-inch round circles from the brownies. Pour some raspberry coulis in the centers of 4 dessert plates and top each with a brownie round. Dollop with heavy cream, garnish with raspberries and serve.
You can crumble the trimmings from the brownie rounds and use them to top ice cream.