Start With The Brownie Base:

1. Preheat The Oven & Prep The Pan. Start by preheating the oven to 325 degrees F. Line an 8×8″ metal pan with parchment paper and set aside. (Use a METAL pan here, or the brownies won’t bake the same)
2. Combine The Dry Ingredients. In a medium bowl, combine the gluten-free flour blend, cocoa powder, salt, and baking powder. Whisk to combine.
3. Melt The Butter, Oil, Sugar & Chocolate. In a medium saucepan, melt butter, oil, sugar and 2/3 cup chocolate chips over low heat. Stir until the butter chocolate chips are melted and remove from heat. (The sugar will likely still look a bit grainy. This is fine!)
4. Beat In The Eggs & Egg Yolk. Whisk in the eggs and egg yolk, one at a time, stirring to fully incorporate before adding the next egg/yolk.
5. Add The Vanilla. Mix in the vanilla extract.
6. Stir In The Dry Ingredients. Add the dry ingredients and stir until just a few small pockets of powder remain.
7. Fold In The Chocolate Chips. Add the second 2/3 cup chocolate chips and fold in with a spatula until just evenly distributed and no more powdery spots remain. Transfer the brownie batter to your prepared baking pan and smooth the surface evenly.
8. Bake The Brownies. Pop the brownies in the oven and bake for 25-27 minutes or until the surface no longer jiggles in the center and the brownies look set around the edges. (Try not to over-bake, as this will dry the brownies out a bit.)
9. Add Marshmallows. Immediately after you remove the brownies from the oven, add the marshmallows and distribute into an even layer. Return brownies to the oven for 2-3 minutes or until the marshmallows are starting to puff and spread. Let the brownies cool down slightly as you mix up the frosting.

Then, Mix Up The Frosting: 

1. Melt Butter, Milk & Cocoa Powder. In a small saucepan, combine butter, milk, and cocoa powder. Whisk until the butter is melted and the mixture is starting to get thick.
2. Finish The Frosting. Remove from the heat and whisk in the vanilla extract, a pinch of salt, and the powdered sugar. Whisk until smooth and no lumps remain. It should look like a thick chocolate glaze–If the frosting is too thick to whisk, add more milk 1/2 teaspoon at a time until you can easily whisk it. If the frosting is very liquid, add more powdered sugar 1 Tablespoon at a time.
3. Pour Glaze & Cool Brownies Completely. Next, pour the warm glaze over the marshmallow layer, doing your best to cover everything evenly. The shape of the marshmallows may start to peek through. This is fine! Cool the brownies totally and completely before slicing of they won’t cut evenly. You can speed this process along by chilling the brownies in the refrigerator.
4. Slice & Enjoy. Cut into slices (9 large squares, 12 rectangles, or 16 small squares) and enjoy. Store leftover marshmallow brownies covered or in an airtight container in the fridge 2-3 days, or in a freezer-safe bag or airtight container in the freezer up to 2 months.