Prepare the baking pan by lining it with parchment paper. Allow the parchment paper to hang over the sides to make for easier removal at the end. Set the pan aside.
Crush the cookies into a large bowl. You can break them up by hand, or bash them around with a rolling pin. Set aside.
Over low heat, add your butter, sugar, and hot chocolate mix to a medium sized pot. Stir to combine. You'll want to let everything melt together. Continue to stir until both the sugar and hot chocolate have dissolved. You'll notice that the mixture will thicken ever so slightly at this point. Remove the pot from the heat.
Beat the egg in a small bowl and whisk in a small amount of the warm chocolate mixture to help temper the egg. Slowly pour the egg back into the chocolate mix while continuing to stir. The mixture will thicken up a bit more to lightly coat the back of a spoon.
Pour in the crushed cookies and stir to combine, breaking up any large bigs of cookies as you stir.
Spoon the mixture into your prepared pan, pressing it down really well with a fork. Place the pan in the fridge to chill for 1 hour before cutting into squares.