Add softened butter, granulated sugar, and brown sugar into a mixing bowl. Mix until the mixture becomes light in color, smooth, and creamy. Scrape the sides of the bowl to ensure even mixing.

Add the egg yolk and vanilla extract to the bowl. Mix thoroughly until the mixture is fully combined, smooth, and slightly glossy with no visible streaks.

Add the flour, baking soda, and salt into the bowl. Mix gently until a soft dough forms. Stop mixing as soon as the flour disappears to avoid overworking the dough.

Add chocolate chips, chopped chocolate, mini marshmallows, and graham cracker pieces. Fold gently using a spatula until evenly distributed throughout the dough.

Cover the dough and refrigerate for 30 minutes. This helps the dough firm up and improves both texture and flavor.

Preheat the oven to 350°F and line a baking tray with parchment paper. Scoop the dough into small portions and place them on the tray with enough space between each piece.

Bake for 9 to 11 minutes, or until the edges are lightly golden and set while the centers remain soft. Marshmallows will puff and slightly brown.

Remove from the oven and allow cookies to cool on the tray for 5 to 10 minutes before serving. This helps them set while keeping the inside soft and gooey.