Eat these straight out of the fridge, and you have brownies with a sweet cookie dough taste. Let them warm up to room temperature, and you have chocolately cookie dough fudge. Awesome either way!
Line a 9x13 pan with aluminum foil and apply nonstick spray. In a microwave-safe bowl, melt the chocolate chips. Let them cool slightly.
Preheat oven at 350-degrees. In a mixing bowl, cream together butter and sugar. Add eggs and vanilla. Stir in the melted chocolate. Combine flour and baking soda, and slowly mix into the wet ingredients. Stir in the walnuts and half of the marshmallows.
Glop it into the ready pan and spread the dough out evenly. Top it with the remaining marshmallows. Bake it for 18-22 minutes, until it passes the toothpick test. Allow to completely cool.
Cookie dough layer
Cream the butter and two sugars, then add the milk and vanilla extract. Gradually add the flour. Add the chocolate chips last.
Dollop the dough on top of the brownies and use an offset spatula to form a thin layer across the top. Set it in the fridge to chill for an hour or so.
Ganache
Set chocolate in a microwave-safe bowl and microwave until you can stir it smoothly. Use an offset spatula to smooth it over the cookie dough layer. Set it at room temperature or chill in fridge to set.
Use the aluminum foil to lift the brownies out for easy cutting. I found it cut smoothest with the brownies flipped over, chocolate on the bottom. Note that the chocolate will still likely shatter some as you cut it.
Brownies are best chilled and stored in the fridge. If kept at room temperature for a while, they soften a lot and become very fudge-like, but firm up again when chilled. It's awesome either way.