Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on two sides for easy lifting.

Make the crust: In a medium bowl, combine graham cracker crumbs, ¼ cup granulated sugar, and 6 tablespoons melted butter. Stir until the mixture resembles wet sand. Press firmly into an even layer on the bottom of the prepared pan. Bake for 8 minutes, until fragrant and just starting to brown at the edges. Remove and set aside.

Make the cookie layer: In a small bowl, whisk together flour, baking powder, and salt. Set aside.

In a large bowl, beat the softened ½ cup butter, brown sugar, and ¼ cup granulated sugar with an electric mixer on medium speed until light and fluffy, about 2 minutes. Scrape down the bowl.

Add the egg and vanilla, beating until combined, about 30 seconds.

Gradually add the flour mixture, mixing on low just until no dry streaks remain. Do not overmix. Fold in 1 cup of the chocolate chips.

Drop spoonfuls of the cookie dough over the pre-baked crust. Use an offset spatula or your fingertips (lightly floured) to gently press the dough into an even layer covering the crust completely. The dough will be sticky; that's fine.

Bake for 18 to 20 minutes, until the cookie layer is golden brown around the edges and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). The center will look slightly underdone — that's your gooey secret.

Remove the pan from the oven. Immediately sprinkle the remaining ½ cup chocolate chips evenly over the hot cookie layer. Let them sit for 1 minute to soften, then gently spread with a spatula. Sprinkle the mini marshmallows and broken graham cracker pieces on top.

Return the pan to the oven and bake for 3 to 4 more minutes, until the marshmallows are puffed and lightly golden. Watch closely — they can brown fast. If you want deeper toasting, switch the oven to broil for the last 30 to 60 seconds, keeping your eyes on them the entire time.

Let the bars cool in the pan on a wire rack for at least 30 minutes before lifting out using the parchment overhang. For cleaner cuts, cool completely (about 1 hour) before slicing into 16 squares. A lightly oiled knife helps prevent sticking.