Preheat the oven to 350 degrees. Butter a 9 inch spring form pan set aside.
Whisk together the flour, sugar, cocoa powder, baking powder and salt in a bowl. Beat together the butter, eggs and vanilla. Add to the dry ingredients and mix until smooth. Stir in the toasted nuts. Pour into the prepared pan.
Bake on a middle shelf of the oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Immediately sprinkle the marshmallows on top and return to the oven for a few minutes to melt the marshmallows. Remove from the oven and carefully spread the chocolate frosting over top.
To make the chocolate frosting mix all ingredients together in a bowl, beating with an electric whisk until smooth and thick. Spread on the hot marshmallows. Let cool completely. Cut into wedges to serve.