Preheat the oven to 350°F (175°C).

Grease the 8x8 inch baking pan with butter or line it with parchment paper.

In a saucepan over low heat, melt the butter.

In a mixing bowl, combine the melted butter and granulated sugar. Add the eggs one at a time, whisking until the mixture is smooth and glossy.

Stir in the vanilla extract, then sift in the flour, cocoa powder, and salt. Mix well.

Spread the brownie batter into the prepared baking pan.

Bake in the preheated oven for 20 minutes.

Remove the brownies from the oven and sprinkle the mini marshmallows on top.

Return the brownies to the oven for an additional 5 minutes until the marshmallows are puffy and golden.