Preheat your oven to 350°F (175°C). Grease an 8×8 inch square baking pan with butter or non-stick spray. For even easier removal, line the pan with parchment paper, leaving an overhang on the sides.
In a large mixing bowl, beat the softened butter and granulated sugar together until the mixture is light and fluffy. This usually takes about 2 to 3 minutes with a hand mixer or stand mixer.
Add the egg and the vanilla extract to the butter mixture. Beat again until everything is well combined and smooth.
In a separate medium bowl, whisk together the all-purpose flour, graham cracker crumbs, baking powder, and salt. Slowly add these dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms.
Divide the dough into two equal halves. Press one half of the dough firmly into the bottom of your prepared baking pan to create an even layer. This will be the sturdy base for your s’mores.
Spread the chocolate chips (or chocolate bar pieces) evenly over the bottom dough layer. Top the chocolate with an even layer of the mini marshmallows. Press them down slightly so they stay in place.
Take the remaining dough and flatten small pieces of it with your hands. Place these pieces over the top of the marshmallows. It is okay if some marshmallows are still peeking through; the dough will spread as it bakes.
Place the pan in the oven and bake for 20 to 25 minutes, or until the top is lightly golden brown. Remove from the oven and let the bars cool completely in the pan before slicing. This helps the marshmallow and chocolate set.