Prep the pan: Heat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang on the long sides for easy lifting. Lightly grease the parchment.

Make the crust: In a bowl, mix graham crumbs, granulated sugar, 1/4 teaspoon salt, and melted butter until it resembles damp sand. Press firmly and evenly into the prepared pan using the bottom of a measuring cup.

Par-bake: Bake the crust for 8–10 minutes until set and lightly fragrant. Remove and cool for 5 minutes while you make the topping.

Mix the cookie layer: In a separate bowl, whisk flour, baking powder, and remaining 1/4 teaspoon salt. In another bowl, beat brown sugar, egg, and vanilla until smooth. Stir dry ingredients into wet to form a soft dough.

Assemble the chocolate layer: Arrange milk chocolate pieces evenly over the warm crust. Sprinkle semi-sweet chips in any gaps for full coverage.

Add cookie dollops: Drop small spoonfuls of the dough over the chocolate. Don’t worry about perfect coverage—little pockets of chocolate peeking through are ideal.

First bake: Return the pan to the oven and bake 12–15 minutes, until the cookie patches are puffed and just set at the edges. The chocolate will be melty underneath.

Top with marshmallows: Remove the pan and scatter mini marshmallows in a thick, even layer. Gently press to help them stick.

Final toast: Switch the oven to broil. Broil the bars on the middle rack for 1–2 minutes, watching constantly, until the marshmallows are golden-brown with a few darker spots. Rotate the pan as needed for even color.

Cool and set: Let the bars cool at room temperature for 30–45 minutes, then transfer the pan to the fridge for 30 minutes to firm up the chocolate for cleaner slicing.

Slice and serve: Use the parchment overhang to lift the slab out. Grease a sharp knife and cut into squares or bars. Sprinkle with flaky sea salt if you like a sweet-salty finish.