Preheat oven to 350°F (175°C) and line an 8-inch square pan with parchment. Whisk butter, sugar, eggs, and vanilla until smooth, then stir in cocoa powder, flour, and salt. Spread into the pan and bake 20-25 minutes. Cool completely.

Trim the leafy tops off the strawberries so they sit flat. Whip the heavy cream with powdered sugar and a splash of vanilla until stiff peaks form.

Cut the cooled brownies into 12 squares or rounds. Pipe a circle of whipped cream onto each piece.

Place a strawberry, pointy side up, on each cream-topped brownie. Add a tiny dollop of cream to the tip and press a mini marshmallow on top. Serve right away or chill up to 2 hours.