Place the mini marshmallows, soft butter, chocolate and water in a heavy-bottomed saucepan over low heat to melt everything; whisking every now and then until melted and combined. Remove from the heat and allow to cool; almost completely, at least 15 minutes.
Using your electric stand mixer fitted with the whisk attachment (on speed 4 or 6) or hand mixer, whip the heavy cream with the vanilla extract until thick and peaks have formed; for about 5-8 minutes. Remove HALF of the cream and place into a bowl and set in the refrigerator (this is for the peanut butter mousse).
Lastly, using a rubber spatula, fold the other half of the fresh whipped cream into the cooling chocolate mixture until you have a smooth, cohesive mixture. Cover with plastic wrap and place into the refrigerator to set for at least one hour.