Preheat the oven to 350°F (175°C). Line muffin tins with 18 paper liners, or spray the cups with cooking spray.

In a large bowl, whisk together the flour, sugar, baking powder, and salt.

Add the softened butter and beat with an electric mixer on low speed until the mixture looks like coarse sand, about 1 minute.

In a separate bowl, whisk together the egg whites, milk, and vanilla. Pour half into the flour mixture and beat on medium speed for 1 minute, then add the rest and beat until smooth, scraping down the bowl as needed.

Fill the liners about two-thirds full. Bake for 18 to 22 minutes, until a toothpick inserted into the center of a cupcake comes out clean.

Cool the cupcakes in the pan for 5 minutes, then move them to a wire rack and let them cool completely before decorating.

In a microwave-safe bowl, melt the candy melts and coconut oil in 30-second intervals, stirring between each, until smooth.

Spoon the melted candy into a sandwich bag or piping bag and snip off a tiny corner. Cut each marshmallow in half crosswise and set the halves cut-side down on parchment paper. Pipe thin, criss-crossing lines over each one, then press two candy eyes into the soft candy. Let set for about 10 minutes.

For the frosting, beat the butter, powdered sugar, and 3 tablespoons of milk on medium speed, gradually increasing to high, until light and fluffy, 2 to 3 minutes. Add more milk if needed, then transfer to a piping bag.

Pipe a swirl of frosting onto each cooled cupcake and gently press a marshmallow mummy on top. Happy Halloween!