2 tablespoons peanut butter plus crushed chocolate-cream sandwich cookies, such as Oreos
2 tablespoons chocolate-hazelnut spread, such as Nutella, plus toasted hazelnuts
3 ounces mixed white, milk, and dark chocolate chunks
To make the brownies: Preheat the oven to 350°F. Butter and line a 9 x 13-inch baking pan with buttered parchment.
In a medium mixing bowl, whisk together the flour, cocoa powder, and salt. Set aside.
In a medium saucepan over medium-low heat, melt the butter to just simmering. Turn off the heat and add the chopped chocolate. Swirl the pan so the chocolate is submerged in the butter and let sit for 3 minutes. Gently whisk until smooth.
In a stand mixer fitted with the whisk attachment, whip together the eggs, sugar, and vanilla on medium-high speed until light and fluffy, about 5 minutes.
Turn the speed to low and slowly add the melted chocolate mixture. Add the flour mixture to the chocolate mixture and stir just until combined. Pour into the prepared pan.
To add the optional toppings, if desired: Warm the peanut butter in the microwave for a few seconds to make it spreadable. Dollop over one-fourth of the brownie batter. Swirl with a skewer, then insert some crushed sandwich cookie pieces.
Warm the chocolate-hazelnut spread in the microwave for a few seconds to make it spreadable. Dollop over one-fourth of the brownie batter. Swirl with a skewer, then sprinkle with some toasted hazelnuts.
Stir half of the mixed chocolate chunks into one-fourth of the batter and top with the remaining.
Leave the last section of the brownie batter plain and simple.
Bake until a tester comes out with moist crumbs, about 35 minutes. If you like your brownies dense and gooey, bake them until the sides puff, but the center is still fudgy when tested.