Preheat oven to 350 degrees Fahrenheit. Line the baking pan with parchment paper so that at least one inch of parchment paper hangs over the edge of the pan. Set aside.
Make the Brownie Batter: Roughly chop chocolate bar and cut butter into chunks then place both into a heat-safe bowl and microwave in 30-second increments until melted (typically takes between one minute and 90 seconds total). Stir together until uniform.
6 ounces dark chocolate, ½ cup unsalted butter
Mix eggs, granulated sugar, brown sugar, and vanilla into chocolate and butter mixture until combined.
3 eggs, ⅔ cup granulated sugar, ½ cup brown sugar, 1 teaspoon vanilla bean paste
In another bowl, whisk together flour, cocoa powder, and fine sea salt, then dredge the walnuts and pecans in this mixture until coated.
½ cup all-purpose flour, 3 tablespoons cocoa powder, ¾ teaspoon fine sea salt
Add the flour mixture to the chocolate mixture and stir until just combined, then pour the batter into the prepared baking dish, spreading it evenly across the pan.
Bake the Brownies: Place pan into preheated oven and bake for 30 minutes or until baked through. You can lift the parchment paper to remove the brownies from the pan in one solid piece for faster cooling, but return them to the pan before adding the peppermint bark topping in step eight.When fully baked, a toothpick inserted into the center of the brownie will come out with moist fudgy brownie crumbs clinging to it but not wet, runny streaks of batter. If it comes out completely clean, the brownies are overbaked. Make the Peppermint Bark Topping: While brownies cool, add white chocolate and dark chocolate to separate bowls and microwave in 10-second ingredients until melted. Stir the peppermint extract into the melted white chocolate.
8 ounces white chocolate, 1 ounce dark chocolate, ¼ teaspoon peppermint extract
Set and Serve the Brownies: Spread the melted white chocolate over top of the cooled brownies, then drop the melted dark chocolate over it in about six spoonfuls. Use a toothpick to swirl the dark chocolate throughout the white chocolate to create a marble effect. Sprinkle crushed peppermint and sprinkles (optional) over top, then lightly tap them into the surface of the chocolate so they stick. Place pan in refrigerator to set for about 35 minutes or freezer for about 15 minutes.The white chocolate layer won't perfectly cover the brownie, but the dark chocolate and decorations will fill in any gaps. If you want a thicker bark layer, double the amount of white chocolate and add 15 minutes to the setting time. 3 tablespoons crushed peppermint candy, 2 tablespoons Christmas sprinkles
Use parchment paper to lift brownie out of pan, then cut into 16 equal pieces. To get perfect cuts without any of the bark cracking, let the brownies come to room temperature.