Preheat oven to 350° F. Grease a 9-inch square baking pan and line with parchment paper.
In a medium bowl, whisk together the flour, baking powder, and salt.
Place cocoa and butter in a microwave safe bowl. Microwave on high, stirring every 30 seconds until smooth.
Stir in the vanilla and sugar.
Add in the eggs, one at a time, stirring after each addition until combined.
Add the dry ingredients to the wet ingredients and stir to combine.
Transfer the batter to the prepared pan and use a rubber spatula to smooth the batter into the corners.
Bake 20-25 minutes until the edges of the brownies start to pull away from the sides of the pan.
Remove from oven and scatter the marshmallows, pecans, and chocolate chips all over the top of the brownies.
Place the brownies under the broiler, set to low, for about 2 minutes or until the marshmallows turn a light brown color.
Take the brownies out of the oven and allow to cool about 20 minutes before refrigerating. Chill about 30 minutes before serving.
Cut the brownies into 16 squares, then cut each square on the diagonal to end up with 32 triangles.
Notes
Line your pan with enough parchment paper so the edges hang over the sides. This will make it easy to remove the brownies from the pan. (Make sure your parchment paper is safe to use under a broiler. If not, you can leave the greased pan unlined.)
Use room temperature eggs. Room temperature eggs combine more easily and completely than eggs straight from the fridge.
Use a 9-inch square baking pan with straight sides, instead of rounded sides, to give the brownies a clean, crisp edge.
When cutting the brownies, spray the knife with non-stick cooking spray. The cookie spray keeps the marshmallows from sticking to the knife.