Preheat oven to 400 degrees F (200 degrees C). In a small sauce melt together the butter and 1/2 cup of the chocolate chips, stirring frequently. Remove from heat and set aside to cool.
In a medium bowl, stir together the melted chocolate mixture with the sugar, eggs and vanilla. Sift together the flour, baking powder and salt, stir into the chocolate mixture. Finally, stir in the chopped nuts and remaining chocolate chips.
Drop dough by rounded teaspoonfuls onto unprepared cookie sheets. Press a miniature marshmallow into the center of each cookie. Bake for 7 to 8 minutes in the preheated oven. Remove from baking sheets immediately to cool on wire racks.