Preheat oven with a rack in the middle of the oven to 350 degrees. Coat a 9 by 13-inch baking pan with nonstick vegetable spray.

Melt the chocolate and butter in a completely dry bowl or in the top of a double boiler set over barely simmering water. In a large bowl, mix together the eggs, granulated sugar, brown sugar and corn syrup until blended. Mix in the chocolate mixture. In a separate bowl, stir the flour, baking powder and salt together with a fork. Mix it into the egg mixture. Stir in the nuts and 1/4 cup of the marshmallows. Scrape the mixture into the prepared pan. Bake for 25 minutes, until barely set. Coat the top with the remainder of the marshmallows and bake for 5 more minutes. Set the pan on a rack to cool. Cut into 1 1/2 by 3-inch bars.