Position a rack in the center of your oven. Preheat the oven to 350F. Lightly coat a 9 by 13 baking pan with cooking spray or line with parchment paper.
Whisk together flours, graham cracker crumbs, baking powder and salt into a bowl. Set aside.
In a second mixing bowl, whisk together melted butter, brown sugar, eggs, and vanilla.
Add dry ingredients and mix until just combined, 5 to 10 seconds. Spread batter into prepared pan.
Bake for about 22-25 minutes, or until the top is set and golden brown. Bars may appear to be not done, but they will set up as they cool.
Remove the pan from the oven. Layer the top with the chocolate bars. Top with marshmallows.
Turn the oven to broil and return the bars to the oven for 1 to 2 minutes or until the marshmallows are lightly toasted. Keep a close eye on them so they don't burn! Set the pan on a wire rack and cool completely before cutting into bars.