Beat butter and sugar at medium speed with an electric mixer until fluffy.
Add eggs, egg yolk, and vanilla and beat until well-blended.
Gradually add rum and mango puree. Beat well.
In a separate bowl, stir together flour, baking powder, baking soda and salt.
Add to batter alternately with sour cream, beating well after each addition.
Mix in mango chunks.
Fill cupcake liners 3/4 full.
Bake at 350 F for 25 minutes.
Mango Meringue Frosting Instructions
Combine the egg whites and sugar in the mixing bowl of a stand mixer.
Set mixing bowl on top of a pot of boiling water.
Whisk until sugar has reached 110 F on a candy thermometer.
Remove mixing bowl from the pot and place on mixer.
Whisk for 10 minutes on medium speed using the whisk attachment.
Whisk on high speed for another five minutes (you should see stiff glossy peaks).
Whisk in mango until just integrated.
Spoon onto cupcakes.
If desired, add more mango on top of the meringue. (As noted above, all of this wet mango will make the meringue deflate and get absorbed into the cupcake.)