Heat the oven to 350°F (175°C). Line a 12-cup muffin pan and prepare a second baking sheet with parchment.
Beat butter and sugar until fluffy. Add eggs and vanilla, then alternate the flour mixture with milk until a smooth batter forms.
Divide among the liners and bake for 17–20 minutes. Transfer to a rack and cool completely.
For the ghosts, beat egg whites with cream of tartar until foamy. Gradually add sugar and continue beating until glossy, stiff peaks develop.
Pipe tall ghost shapes onto parchment. Bake at 225°F (110°C) for about 60–75 minutes, then switch off the oven and let them cool inside.
Pipe generous vanilla frosting swirls onto the cupcakes. Use melted dark chocolate to make tiny eyes and mouths on the cooled meringues.
Nestle one ghost into each frosting swirl immediately before serving to preserve the meringues' crisp texture.