Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan with butter or non-stick spray.
In a large mixing bowl, whisk together the sugar, flour, cocoa powder, baking powder, and salt until well combined.
Add the melted butter, eggs, and vanilla extract to the dry ingredients. Mix until the batter is smooth and well blended.
Fold in the chocolate chips and chopped pecans, if using. Pour the batter into the prepared baking pan and spread it evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Remove the cake from the oven and immediately sprinkle the mini marshmallows over the top. Return the cake to the oven for an additional 5 minutes, or until the marshmallows are puffed and slightly golden.
Allow the cake to cool in the pan for at least 15 minutes before slicing and serving.