Preheat your oven to 350°F (175°C). Grease an 8-inch square baking pan and line it with parchment paper.

In a mixing bowl, whisk together melted, cooled unsalted butter and packed brown sugar until smooth. Add the egg, egg yolk, and vanilla extract, stirring until glossy.

In a separate bowl, whisk together plain flour, baking powder, and salt until evenly combined. Add dry ingredients to wet mixture to create the batter.

Gently fold the dry ingredients into the wet mixture until just combined, with the batter remaining thick and slightly lumpy.

Fold in the biscuit or graham cracker crumbs and dark chocolate chips until evenly distributed.

Spread the batter into the prepared baking pan and bake for 25-30 minutes until slightly underdone with a few moist crumbs on a toothpick.

Remove from oven and allow to cool for about 10 minutes before adding the marshmallows.

Evenly sprinkle mini marshmallows across the top and press them in gently.

Toast the marshmallows with a kitchen torch or in the broiler until golden brown and bubbly, watching closely to avoid burning.

Let the blondies cool completely for about 1-2 hours before lifting them out and slicing into squares.