Preheat the oven to 350°F (175°C) and line an 8x8 inch square baking pan with parchment paper.
In a large mixing bowl, sift together the flour and cocoa powder. Whisk in the sugar, baking powder, and salt.
Add the plant milk, melted coconut oil, and vanilla extract to the dry ingredients. Stir until a thick, smooth batter forms.
Fold in half of the vegan chocolate chips.
Transfer the batter to the prepared pan and spread evenly with a spatula.
Press the graham cracker shards and the remaining chocolate chips into the top of the brownie batter.
Bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs.
Remove the pan from the oven and immediately top with the vegan mini marshmallows.
Place the pan under the broiler for 1 to 2 minutes, watching closely, until the marshmallows are puffed and golden brown.
Allow the brownies to cool completely in the pan for at least 1 hour to ensure a clean slice.