These S'mores Brownies start with a chewy blondie brownie layer that tastes like a buttery, honey kissed graham cracker followed by a thick layer of creamy dark chocolate and finished with a toasted billowy smooth marshmallow meringue.
In a medium bowl whisk together melted butter, brown sugar and honey. Add egg, vanilla and all dry ingredients (flours, baking soda, baking powder, and salt); stir to combine. Pour into prepared pan and spread into an even layer; bake for 25 minutes. Remove from oven and cool completely.
For Creamy Chocolate Layer
Add butter and chocolate chips to a heatproof bowl set over (but not touching) simmering water. Stir until melted and glossy. Spread mixture over cooled Blondie Brownies and refrigerate until cold and set.
For Marshmallow Meringue Layer
Preheat broiler.
Add lightly beaten egg whites and sugar to a heatproof bowl set over (but not touching) simmering water. Whisk constantly until mixture is warm and sugar has dissolved, about 3 minutes.
Transfer egg white mixture to bowl of a stand mixer fitted with whisk attachment. Add vanilla and cream of tartar and whip at medium speed until firm. Increase speed to high and whip until meringue is glossy and stiff, about another 5 minutes.
Spread meringue evenly over blondie brownies cold chocolate layer, swirling the top decoratively with spoon or spatula. Broil 8-inches from heat for about 1 minute to toast meringue - watching constantly so it doesn't burn.
With a thin, sharp knife cut brownies into 16 2-inch squares. Have a bowl of hot water and a paper or cloth towel to wipe the knife clean between each cut. Serve.