For "mash" you can use the verb "amassar". But just out of curiosity, what "plantains" mean?

@adilson it means banana da terra.

Thanks for your help

When you're ready, send me that recipe, please.

Adilson is right, "to mash" means "amassar". But if you're giving a recipe, you're might be saying you're going to mash the plaintains up with butter, with would translate into the verb "misturar". So, if you're going to crush them to a pulp, you'll use "amassar"; if you're mixing them, you'll use "misturar". Hope I didn't make a mess out of it :P