Lately my girfriend is very open to play with pies!!

However, I'm not able to find a good recipe!!

I'm using Thermomix to whip eggwhites basically (with lots of sugar)... however, it's too runny... I tried today meringue buttercream, adding butter... but has been spoiled... like water...

Any one can help me with this? I want to use eggwhites, sugar, butter,... nothing dificult to find...

Thanks a lot in advance!!

A bit labour-intensive especially if you don't have a stand mixer, but red velvet frosting is very thick and stable. You can add all kinds of things to it too -
fruit purees, melted chocolate, cream cheese... or you could probably slacken it by beating in some regular cream. Recipe here, which could be scaled up: http://www.epicurious.com/recipes/food/views/creamy-vanilla-frosting-241564

As for meringue, it's probably always going to collapse unless it's cooked - an easy way to make gooey marshmallow-like meringue is to line a cake pan with baking paper, spread your sugar in a layer and bake it in the oven until it's hot. Then it will cook the egg whites as it is gradually beaten in.

Edit - Just googled what a Thermomix is. I'm guessing your problem is that when you add butter, the machine is melting it and making soup. My advice would be to make your cooked meringue, and allow everything to cool before beating in softened butter without any additional heat.