I make fluffy American buttercream very often, and here are the tips I could give!
1. Your recipe is saying to add the other liquid ingredients (extracts and cream) to the butter first then add the powdered sugar. I wouldn't do this. The amount of liquid you need to add to your buttercream will very depending on temperature of the butter, and of course how much powdered sugar you are using.
2. Make sure the butter is room temperature- where you can indent it with your finger- it is not too melty, but definitely not cold or hard. It should at least be malleable when squeezed between your fingers/hands. Your recipe says only to leave your butter out for 15 minutes- you said it is hot and humid where you live, although I have never heard of butter coming to room temperature that fast still in stick form. A trick for having it come to room temp more quickly and evenly- cut up the butter into cubes. This will drastically cut down the time, but do not focus on any number of minutes, it has to be the right consistency, which will vary.
3. Sifting the powdered sugar is super important, which I see your recipe states to do, so definitely continue to do this and make sure there are no little hard pieces in the sifted powdered sugar (as in humid/moist environments powdered sugar can clump and become hard. Make sure you are using a fine mesh sifter/strainer, and if/when you have hard pieces of powdered sugar left after sifting, you discard them- don’t dump them back in or try to force them through the sifter.
How I would attempt to make your recipe-
With completely room temperature butter (better slightly warmer than colder) beat in a mixer until creamy, then slowly add powdered sugar little by little. If buttercream starts to get very thick or crumbly, alternate with wet ingredients. Start with your extracts, and I suppose food coloring if you are wanting to add that, then start adding the heavy cream. Only little by little! Once all the powdered sugar is added, if still to thick, add heavy cream teaspoon by teaspoon until it is the right consistency. Just remember to add all the ingredients slowly- and don’t over whip! Have it on a low-setting and only increase speed every now and then to aerate and incorporate ingredients well. Pinch of salt can be added at any stage.
Hope this helps!!!! Good luck