Preheat your oven to 350F and line a baking sheet with parchment paper.
In a mixing bowl, combine the flour, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, beat the butter, sugar, egg, and vanilla extract together until well combined.
Stir in the crushed Oreo cookies and white chocolate chips if using.
Use a cookie scoop or spoon to form the dough into 24 balls and place on the prepared baking sheet.
Bake for 12-15 minutes, or until the edges are golden brown.
Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
While the cookies are cooling, make the filling by mixing together the butter, powdered sugar, and vanilla extract. Once the cookies have cooled, spread the filling on the bottom of half of the cookies and top with the remaining cookies to make sandwiches.