• Preheat the oven to 350 degrees and line an 8 x8 inch baking pan and line with parchment paper. Make it long enough so there is about a ½ overhang on each side so that you can lift the brownies out easily once they have cooled.

  Do not grease. There is enough oil in the brownies that greasing would be too much.
• Chop the 8 ounces of chocolate into tiny pieces. Place the chopped chocolate in a microwave safe bowl with the 6 tablespoons unsalted butter and 2 tablespoons vegetable oil. 

  Heat in the microwave in 15 second increments and stir after each time until the chocolate is completely melted and silky smooth. Set aside.
• Pour the ¼ cup milk and the 2 teaspoons white vinegar into a small bowl. Stir and let sit 5 minutes. Stir again. It will appear chunky and that's normal. Set aside.
• In a small bowl, using a spoon, mix together the 4 rounded tablespoons cornstarch and the 6 tablespoons water until thin and watery. It will be thick and tacky at first.
• Into the melted chocolate and butter, whisk in the cornstarch water until fully combined.
• Then whisk in the 1 cup granulated sugar, 1 teaspoon pure vanilla extract, 1 teaspoon baking powder and ½ teaspoon kosher salt until combined.

  Lastly, add in the milk vinegar mixture and whisk till combined.
• Using a spatula, gently fold in the 5 tablespoons cocoa powder and 1 ¼ cups multipurpose gluten free flour until everything is just combined.

  The batter will be super thick and sticky. 
• Spoon half the batter into the baking dish and use your hands to press it down into the shape of the pan.

  Gently press 16 gluten free/vegan chocolate sandwich cookies into the batter.

  Then take the other half of the batter and press it on top of the chocolate sandwich cookies, covering all of them. You should end up with a smooth even layer of batter.
• Bake the brownies in the center rack of the oven at 350 degrees Fahrenheit for 40 minutes.

  Let the brownies cool completely before frosting.

Make the Frosting

• While the brownies are baking, make the frosting. In the large bowl of an electric mixer or using a hand held mixer, beat the 4 tablespoons butter or vegan baking stick until smooth and creamy. About 2 minutes.

  With the mixer on very low speed, slowly add in the 2 cups confectioners' sugar. Add in the ½ teaspoon vanilla and beat until combined. While the mixer is on, slowly add in the 2 tablespoons water a little at a time until you get the desired consistency. You may not need to use all the water.

  Using a spatula, gently fold in the 6 broken chocolate sandwich cookies until just combined. If you over-mix them the frosting will become grey instead of white.
• Once the brownies have completely cooled, keep them in the pan and frost the top completely with all of the buttercream frosting.

  Pop the brownies (still in the pan) in the fridge for at least 15-20 minutes. You want the frosting layer to become hard for the next step.

Make the Ganache

• When the brownies are ready to come out of the fridge, make the ganache.

  Place the 1 cup chocolate chips and the ½ cup butter or vegan baking stick in a microwave safe bowl. Heat in 15 second increments, stirring after each time, until the melted and silky smooth.
• Take the brownies out of the fridge, and pour the liquid ganache on top of the frosting while still in the pan.

  Quickly break up 3 more chocolate sandwich cookies and scatter them over the top of the brownies while gently pressing them into the ganache so they stick.

  Pop the brownies back in the fridge until the ganache hardens.
• Thaw the brownies on the counter for at least 10 minutes prior to cutting. Lift the brownies out of the pan using the parchment overhang and slice the brownies into big squares, tiny squares, large rectangles or small rectangles...whatever your heart desires.