Beat the butter and sugar until creamy. Add the vanilla extract and then the eggs, one at a time. Wait until each egg is fully incorporated before adding the next.
Finally, add the flour, cocoa powder, baking powder, and salt. Mix until the dry ingredients are just combined.
Line a muffin tin with cupcake liners and divide the batter evenly among them. I like to use an ice cream scoop - it makes portioning neat and easy.
Bake the cupcakes at 180 °C/350 °F (conventional oven) for about 18 minutes.
Let the cupcakes rest in the pan for a few minutes before transferring them to a wire rack to cool completely.
Prepare the buttercream according to the recipe. At the last step, mix in the hazelnut spread and cocoa powder. Taste and adjust with more cocoa powder if you want a stronger chocolate flavor.
Separate the Oreos from their filling - do this for both the mini and regular Oreos. Break the regular Oreos neatly in half.
Fill a piping bag fitted with a round tip and pipe a generous swirl of buttercream onto each cupcake. Insert two Oreo halves into the swirl to look like bat wings, placing them slightly behind the center.
Attach candy eyes to each mini Oreo (I used the leftover Oreo filling as glue, but buttercream works too), then press them onto the front of the swirl to make the bat faces.