Preheat Oven: Preheat oven to 350°F (175°C). Grease or line an 8x8-inch baking pan with parchment paper.
Mix Wet Ingredients: In a medium bowl, whisk together melted butter, sugar, eggs, and vanilla until smooth.
Add Dry Ingredients: Sift in cocoa powder, flour, salt, and baking powder. Gently fold until combined, being careful not to overmix.
Fold in Marshmallows: Gently fold in 1 cup of mini marshmallows and chocolate chips or nuts if using.
Bake: Pour batter into prepared pan and spread evenly. Bake for 25–30 minutes, until a toothpick inserted near the center comes out mostly clean (marshmallows may slightly caramelize).
Optional Topping: Sprinkle extra marshmallows on top in the last 5 minutes of baking and broil for 1–2 minutes to toast them lightly. Watch carefully to avoid burning.
Cool and Serve: Allow brownies to cool in the pan for at least 10 minutes before cutting into squares. Serve warm or at room temperature.