Prep the pan and oven. Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving an overhang for easy lifting. Lightly grease the parchment and sides.

Melt the butter. In a microwave-safe bowl or small saucepan, melt the butter until just liquid, not sizzling. Let it cool for 2–3 minutes so it doesn’t scramble the eggs.

Whisk the wet ingredients. In a medium bowl, whisk the melted butter with the sugar until glossy. Add the eggs and vanilla, whisking until the mixture looks thick and slightly lightened, about 30 seconds.

Combine the dry ingredients. In a separate bowl, whisk cocoa, flour, baking powder, and salt until no streaks remain.

Make the batter. Add the dry mix to the wet and stir with a spatula just until combined. If using chocolate chips inside the brownies, fold in 1/4 cup now.

Bake the base. Spread the batter evenly in the pan. Bake for 18–22 minutes, until the top is set and a toothpick inserted near the center comes out with a few moist crumbs. Don’t overbake.

Add the marshmallows. Remove the pan and quickly scatter the mini marshmallows in an even layer over the hot brownies. Return to the oven for 2–3 minutes, just until the marshmallows puff and start to look glossy at the edges.

Optional drizzle. While the marshmallows set, melt the remaining 1/4 cup chocolate chips with 1 tablespoon oil in the microwave in 20-second bursts, stirring between each, until smooth. Drizzle over the warm marshmallow layer.

Cool before cutting. Let the pan cool on a rack for at least 45–60 minutes. For cleaner cuts, chill for 20 minutes after cooling. Lift out using the parchment and cut with a greased knife.