PREHEAT THE OVEN TO 350 F.
USING A STAND MIXER OR LARGE BOWL AND HAND MIXER, BEAT BOTH SUGARS AND BUTTER UNTIL CREAMY. ADD IN THE EGG AND VANILLA, BEAT AGAIN.
POUR IN THE FLOUR, SALT, AND BAKING POWDER AND MIX UNTIL COMBINED.
STIR IN THE GRAHAM CRACKER CRUMBS WITH A SPOON OR SPATULA UNTIL EVENLY MIXED.
SET OUT 2 SQUARES OF PARCHMENT PAPER, SPLIT THE DOUGH INTO 2 EVEN PIECES.
PLACE THE PARCHMENT PAPER SQUARE INTO AN 8X8" DISH AND FLATTEN OUT THE COOKIE DOUGH IN THE BOTTOM OF THE PAN. LIFT OUT AND SET ASIDE, THIS COOKIE HALF WITH BE THE TOP LAYER.
REPEAT WITH THE SECOND PIECE OF PARCHMENT PAPER AND COOKIE DOUGH HALF, FLATTEN OUT INSIDE THE BOTTOM OF THE PAN UNTIL EVEN. THIS IS THE BOTTOM BASE OF THE BARS.
ADD THE CHOCOLATE BARS ON TOP OF THE COOKIE BASE LAYER, HALF MILK CHOCOLATE AND HALF DARK CHOCOLATE. USE YOUR PREFERENCE OF CHOCOLATE HERE, WE JUST LIKE HALF AND HALF!
MELT THE PEANUT BUTTER IN THE MICROWAVE SLIGHTLY AND POUR OVER THE MILK CHOCOLATE HALF. PEANUT BUTTER IS OPTIONAL; 1/4 CUP FOR HALF OR 1/2 CUP FOR THE WHOLE PAN. OR SKIP THE PEANUT BUTTER ENTIRELY, IF YOU PREFER!
SPOON MARSHMALLOW FLUFF ON TOP AND SMOOTH GENTLY WITH THE BACK OF A METAL SPOON.
TAKE YOUR TOP COOKIE DOUGH LAYER AND GENTLY FLIP IT UPSIDE DOWN OFF THE PARCHMENT PAPER ON TOP OF THE MARSHMALLOW FLUFF LAYER. GENTLY PULL OFF THE PARCHMENT PAPER USING YOUR FINGERS TO KEEP THE COOKIE DOUGH SEPARATED AS THE TOP LAYER OF THE BARS.
BAKE BARS FOR 30 MINUTES UNTIL GOLDEN BROWN ON TOP AND ON THE EDGES.
AFTER REMOVING FROM THE OVEN AND COOLING, SLICE INTO 16-24 BARS BASED ON YOUR PREFERENCE. THESE ARE BEST CUT AFTER PLACING IN THE FRIDGE OR LETING SET FOR ABOUT 4 HOURS.