Start by preheating your oven to 350°F (175°C). Prepare a 9x13-inch baking pan by lining it with parchment paper and coating it with nonstick spray for easy removal.

In a large microwave-safe bowl, melt the butter in 30-second increments, stirring in between until fully melted. Add the chopped semisweet chocolate to the melted butter, stirring until the chocolate is fully melted and smooth. If needed, microwave for an additional 30 seconds. Whisk in the cocoa powder until combined. Set aside to let the mixture cool slightly.

In a separate large bowl, whisk together the eggs and brown sugar until the sugar has fully dissolved, about 90 seconds of vigorous mixing. Add the powdered sugar and vanilla extract, whisking until the mixture is smooth and creamy.

Next, pour the cooled chocolate-butter mixture into the egg mixture, whisking to combine. Using a rubber spatula, gently fold in the flour and kosher salt until just incorporated. Finally, fold in the chocolate chips.

Transfer the brownie batter into the prepared baking pan and smooth the surface. Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out mostly clean. Let the brownies cool in the pan for at least 90 minutes.

While the brownies are cooling, prepare the tombstone cookies. Line a small baking sheet with parchment paper and arrange the oval-shaped cookies on top. In a zip-top bag, melt 1 to 2 tablespoons of chocolate chips by microwaving in 30-second intervals. Snip off a small corner of the bag and carefully pipe the word "RIP" onto each cookie. Place the cookies in the refrigerator to set.

To make the pistachio whipped cream, add the cold heavy cream to a stand mixer fitted with the whisk attachment. Whisk on high speed until stiff peaks form. Gradually add the pistachio pudding mix, lowering the speed to mix the pudding into the cream. Use a rubber spatula to scrape down the sides of the bowl, ensuring everything is well-combined.

Assembling the trifle is the final step. Slice the cooled brownies in half. Crumble one half and layer it at the bottom of a trifle dish, pressing it down slightly. Spread half of the pistachio whipped cream over the brownie layer. Crumble the remaining brownie half and sprinkle it over the whipped cream. Top with the rest of the pistachio whipped cream.

For the finishing touches, scatter the crushed Oreos across the surface of the whipped cream, positioning the tombstone cookies on top. Garnish with halved gummy worms for a fun and spooky touch. Feel free to add any other decorations to suit your Halloween theme.