Attempt at Robuchon mashed potatoes - troubleshooting?

I tried making Robuchon style mashed potatoes (recipe here: https://www.foodandwine.com/recipes/mashed-potatoes-kind-robuchon-style). Followed the steps all the way until incorporating butter bit by bit — the first three quarters of butter seems to incorporate well. However when I added the last quarter of butter, it just becomes melted and does not incorporate to the potatoes. I just kept on stirring it, adding the heavy cream thinking it will help them incorporate, but to no avail. It just melted, separate from the potato puree.

What did I do wrong?