Preheat the oven to 350F (180C or 160C on a fan-forced oven). Lightly grease a 9x13 inch (23x33 cm) baking pan, or line with parchment paper so that there's an overhang around the edges.
Whisk together the melted butter and sugar in a large bowl. Continue whisking until the butter no longer looks separate from the brown sugar.
Whisk in the eggs and vanilla extract until there are no pieces of egg yolk or egg white.
Gently stir in the flour and salt. If you see lumps of flour, whisk the batter until they disappear.
Stir in the chopped Oreos. It's totally ok and expected that some will fall apart.
Spoon the batter into the prepared pan and smooth the top.
Bake in the preheated oven for 25-30 minutes, or until the top looks set and an inserted toothpick comes out clean or with a few moist crumbs.
Let the bars cool fully before slicing for cleaner, easier cuts.
If the pan is lined with parchment paper, lift the bars out of the pan using the overhang of the parchment from the long edge of the pan. Place the bars on a cutting board and slice using a sharp knife.