Preheat the oven and mix the dry ingredients

• Preheat the oven to 180 °C (350 °F).
• In a large bowl, whisk together the flour, graham cracker crumbs, baking powder and salt, then set aside.

Cream the butter and brown sugar

• In the bowl of a stand mixer, cream the butter and brown sugar until light and fluffy.

Add the eggs and vanilla

• Add the eggs one at a time, fully incorporating each before adding the next.
• Add the vanilla extract and mix until just incorporated.
• Stop the mixer and scrape down the sides of the bowl well, then mix again on medium speed for 15–30 seconds.

Incorporate the dry ingredients

• Turn the mixer to low and slowly add the dry ingredient mixture, mixing until just incorporated.

Layer the dough, chocolate and marshmallows

• Line a 9x13 baking pan with parchment paper and grease it with the residual butter from the butter wrappers.
• Press three quarters of the cookie dough into the bottom of the pan until flat and smooth.
• Sprinkle over the chocolate chunks, then layer the mini marshmallows on top.
• Dollop the remaining cookie dough all over the top so the marshmallows and chocolate still show through.

Bake the bars

• Bake until the marshmallows and cookie dough are golden brown.

Cool and cut

• Transfer the pan to a cooling rack and let cool completely.
• Remove from the pan, trim the edges and cut into square bars.