Preheat the oven to 350 degrees F.
Line a 12-cup muffin pan with paper liners. Whisk together the flour, baking powder and salt in a bowl. Beat the butter and sugar in a bowl with an electric mixer on medium-high speed until fluffy, about 4 minutes. Beat in the eggs, one at a time, scraping down the bowl as needed. Beat in the vanilla. Reduce the mixer speed to medium-low; beat in half of the flour mixture, then all of the milk, then the remaining flour mixture until just combined.
Divide the batter among the muffin cups, filling each three-quarters full. Bake, rotating the pan halfway through, until a toothpick inserted into the center of a cupcake comes out clean, 18 to 20 minutes. Transfer the pan to a rack and let cool 5 minutes, then remove the cupcakes to the rack to cool completely. Using a serrated knife, trim off the tops of the cupcakes so that they are flat.
Place the dark chocolate and white chocolate chips in two separate bowls. Add 4 teaspoons coconut oil to the dark chocolate and the remaining 1 1/2 teaspoons coconut oil to the white chocolate. Melt each bowl of chocolate in the microwave at medium power, stirring every 30 seconds, until just liquid and smooth.
Working with one cupcake a time, spoon 1 heaping tablespoon of the dark chocolate on top, spreading it evenly over the top with the back of the spoon. While the dark chocolate is still liquid, spoon 1 teaspoon of the white chocolate in the center of the cupcake. Drag a toothpick or skewer through the center of the white chocolate outward to form a "tail." Repeat with the remaining cupcakes.
Rewarm the dark chocolate until liquid again, if necessary. Scrape the chocolate into a resealable plastic bag, snip off one corner and pipe small dots of dark chocolate on the white chocolate ghosts to form 2 eyes. Pipe a third dot twice as big to form a mouth. Let the chocolate set for 10 minutes before serving.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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