Make the brownies: In a medium bowl, whisk together the flour, cocoa powder, and salt. Set aside.
In a separate, large mixing bowl, whisk together the melted butter, sugar, eggs, and vanilla extract until smooth.
Add the dry ingredients to the bowl with wet ingredients and mix until just combined.
Spread the batter evenly into the prepared pan.
Bake in the preheated oven for 25–30 minutes, or until the edges are firm and the top is shiny.
Let the brownies cool slightly, then use a skewer or fork to poke holes all over.
Make the pudding: Pour the cold milk into a large bowl. Add the pudding mix and whisk for 2 minutes (it should still be more liquid than set).
Immediately pour 1 ½ cups of the pudding mix over the brownies. Reserve the remaining ½ cup pudding.
Once the brownies are totally cool, make the frosting: Beat together the melted butter, powdered sugar, cocoa powder, and reserved ½ cup pudding until smooth.
Spread the frosting evenly over the cooled brownies.
Sprinkle the chopped Oreos over the frosted brownies. Drizzle chocolate syrup over the chopped Oreos, if desired.
Remove the brownies and the parchment paper from the pan, cut into squares, and serve.