Preheat the oven to 350°F. Line the bottom of muffin cups with parchment paper or waxed paper. Grease the paper with nonstick spray and set aside.
Place 1 teaspoon of melted butter into each cupcake well.
Sprinkle each well with 1 tablespoon of brown sugar.
Put 4 pineapple chunks in a flower pattern over the butter and brown sugar.
Press a maraschino cherry into the center of each pineapple "flower". Set aside.
In a medium bowl, combine the dry ingredients, whisking until combined.
In a stand mixer fitted with the paddle attachment, cream the butter and sugar and beat on medium speed until well combined, about 3 minutes.
Add the egg whites, in three additions, beating each addition until well combined, about 1 minute.
Add the oil and mix until combined, about 1 minute or more.
In a separate bowl, whisk together the milk, vanilla, guava purée and food colouring.
With the mixer on low speed, alternate adding the wet and dry ingredients (starting with the dry ingredients), mixing each until combined before adding the next addition.
Pour the mixture into the cupcake tins (about ¼ cup each) and bake until a toothpick inserted into the center comes out clean 18-20 minutes. Let the cake cool in the pan for 5 minutes before turning out onto a parchment-paper lined surface to cool completely.