Place the pecans in a single layer on a baking sheet and bake at 350 F for about 8 minutes, or until toasted.
Grease a 9 x 9" baking dish.
Melt the butter and chocolate in a glass bowl in the microwave, stirring every 30 seconds until it's melted and smooth.
In a separate bowl, whisk together the sugar, flour and cocoa powder, then whisk in the melted chocolate then whisk in the eggs, vanilla and salt.
Pour the batter into the prepared baking dish and bake for about 20 minutes. Remove from the oven and sprinkle on the marshmallows, then return to the oven for about 10 more minutes, until the marshmallows have puffed up and turned golden brown.
Drizzle the cake with the icing and sprinkle with the pecans.
for the icing:
Stir the butter, cocoa powder and milk together in a medium saucepan over medium heat until the butter is melted. Cook, stirring often, for about 2 - 3 minutes, or until slightly thickened, then remove from heat.
Use an electric mixer to gradually beat in the powdered sugar and vanilla until smooth.