Melt the butter in a large pot over medium-low heat. Add the Gorgonzola and stir until melted. Add nutmeg, white pepper, and onion powder. Add brandy and flambe. Whisk in the heavy cream until smooth. Bring to a boil and add the walnuts. Divide gnocchi among 4 bowls and serve sauce immediately over top. Garnish with 1 prosciutto slice per bowl.