It comes from the British version of Thermomix's Everyday Cookbook. If you don't have a thermie, just steam or boil the potatoes then mash by hand or pass through a potato ricer.
I've tweaked the recipe a little, so here is my version
750g floury potatoes, cut into 2cm cubes
1 tsp salt
1tsp vegetable stock concentrate or stock powder (see here for recipe)
Add water to jug and place potatoes in steamer basket. Place steamer basket in to the thermie jug and cook on soft spoon at varoma temperature for 20 - 25mins or until soft. If cooking on stove top, steam or boil potatoes until well cooked. Do not try to mash undercooked potatoes, they'll turn to glue!
When potatoes are cooked, remove the basket and drain the potatoes. Remove the water from the jug and attach the butterfly whisk. Place potatoes back in the jug and add the remaining ingredients. Whisk on speed 4 for 2-10 seconds.
The kids love these! You could also add your own favours or seasoning such as roasted garlic or parmesan cheese for a change. Enjoy :-)