Preheat the oven to 350F (180C). Line a 12-cavity muffin pan with muffin papers. You will get about 16 cupcakes in total, so you will need to line 1 full pan and add 4 papers to the middle of a second pan.
In a medium bowl, whisk together the melted butter, egg whites, milk, sour cream and vanilla extract. Keep whisking until you no longer see pieces of egg. If the mixture curdles a little, that's ok.
In a separate large bowl whisk together the flour, sugar, baking powder, baking soda and salt until no lumps remain.
Make a well in the middle of the dry ingredients and pour the wet ingredients into the well. Whisk together (by hand) until you no longer see lumps of flour. I typically do a few stirs with a rubber spatula first, then whisk until smooth.
Carefully fold in the chopped Oreo cookies - you can choose to use just the pieces or use the pieces and the crumbs.
Spoon the batter into the lined muffin tins - filling each cupcake liner about 2/3 to 3/4 full. They will each use about 3 tablespoons of batter. Do not try to fit all of the batter into 12 cupcake liners.
Bake 1 muffin tray at a time in the middle of the preheated oven. The cupcakes will bake for about 17-20 minutes, or until an inserted toothpick comes out clean or with a few cupcake crumbs and the tops feel slightly firm and springy to the touch.
Cool the cupcakes in the pan for at least 10 minutes, before carefully transferring to a wire rack to continue cooling.