Preheat the oven to 350°F and lightly grease a 9×9 square baking pan with butter or baking spray and line with parchment paper. Set aside.
In a large bowl, sift together the flour, cocoa powder, and salt. Set aside. Place the Oreos into a large Ziploc bag and roughly crush them. Alternatively, you can chop them on a cutting board. Reserve around ½ to ¾ cup of Oreo pieces for topping.
In a microwave-safe bowl, heat the butter in 30 second increments until melted and very hot (be careful not to let the butter pop or explode). Add the granulated to the butter, whisking until well incorporated and very smooth, around 2 minutes.
Crack the eggs in one at a time followed by the egg yolk, then add the peppermint extract and vanilla. Whisk the mixture until it is smooth and has lightened slightly in color.
In a small bowl, heat the chocolate in the microwave, stirring every 30 seconds until melted. Alternatively, you can melt the chocolate over a double boiler. Allow the chocolate to cool slightly, then pour the melted chocolate into the egg and sugar mixture and whisk well.
Pour the dry ingredients into the wet ingredients, then add in the Oreos, chopped peppermint bark or chocolate chips, and crushed peppermint, then fold everything together until just combined.
Pour the brownie batter into the prepared pan and top with the reserved Oreos and extra peppermint bark or crushed peppermint pieces, if desired. Place the pan in the oven and bake for 35-40 minutes.
The brownies are done baking once puffed and set in the middle; if you insert a toothpick into the center of the brownie, a few moist crumbs should stick to it, but not raw batter (the brownies will continue to cook and set while they cool).
Remove the pan from the oven and top the brownies with flaky salt, if desired. Let them cool completely before lifting out of the pan, slicing, and serving.