Preheat your oven to 350°F (175°C). Prepare a 9x13-inch baking pan with parchment paper and light grease it.

In a large mixing bowl, whisk together melted unsalted butter, granulated sugar, and brown sugar until smooth. Add cold eggs and pure vanilla extract, mixing thoroughly.

In a separate bowl, sift together unsweetened cocoa powder, all-purpose flour, and salt. Gradually whisk this into the wet mixture, taking care not to overmix.

Fold in coarsely crushed Oreo cookies until evenly distributed.

Pour the batter into the prepared pan and bake for 25-30 minutes. Allow cooling completely on a wire rack for 1-2 hours.

For the frosting, beat softened unsalted butter until fluffy. Gradually add sifted powdered sugar, vanilla extract, and milk, mixing until smooth. Incorporate green gel food coloring.

Spread green frosting over the cooled brownies and decorate with Halloween Oreos, edible candy eyes, and assorted sprinkles.

Refrigerate decorated brownies for about 30 minutes, slice into squares, and serve. Store leftovers in an airtight container.