Heat oven to 350°F (325°F for dark or nonstick pans). Place foil or paper baking cup in each of 24 regular-size muffin cups.
In a large bowl, beat all cupcake ingredients with an electric mixer on low speed for 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Divide batter among muffin cups.
Bake 16-18 minutes or until the toothpick inserted in the center comes out clean. Cool 10 minutes; remove cupcakes from pans to cooling racks. Cool completely.
While the cupcakes are cooling make the frosting. In a large bowl, beat the butter with an electric mixer on low speed until light and fluffy. Gradually add powdered sugar, a few tablespoons at a time, beating until combined. Scrape down the bowl then add in the coconut milk and the rum. Beat until blended light and fluffy.
Using a small ice cream scoop, scoop the frosting on top of each cupcake. Using a spatula, spread the frosting in an even layer across the top of each cupcake. Roll the sides of each cupcake in the toasted coconut. Lastly, top each cupcake with the cubed pineapple and an umbrella.