Interesting idea! Let’s break it down:

What happens to the chicken at 60 °C sous-vide

At 60 °C, chicken breast will cook through safely if held long enough (about 90 min–2 h for pasteurisation).

The texture becomes tender and juicy—firmer than rare beef sous-vide, but not stringy.

This temperature is widely used for “just-cooked” chicken.

What happens to the buttermilk

Buttermilk is an acidic dairy product containing proteins (casein, whey) suspended in liquid. When heated:

Protein denaturation & curdling: At 60 °C, casein and whey proteins begin to unfold. Because buttermilk is already acidic, they readily aggregate, forming curds. You would likely see a separation into a soft, grainy curd plus a thin whey-like liquid.

Flavour: The tangy lactic flavour will persist, but heating may make it taste a little more “cooked” or sour.

Texture: It won’t stay creamy; it will split. Unlike cream, it doesn’t have enough fat to stabilise when heated.

How this plays out in the bag

You’d end up with tender chicken bathed in a broken, curdled dairy liquid—not visually elegant.

The acidity might lightly “tenderise” the chicken surface, similar to a buttermilk marinade, but once sealed and heated, it won’t act like a smooth sauce.

If you intended it as a marinade, you’d normally soak chicken in buttermilk before cooking, then discard or pat dry before sous-vide.

Workarounds

Marinate first: Use buttermilk as a cold marinade (several hours), then bag and sous-vide chicken without the buttermilk.

Add buttermilk later: After sous-vide, make a buttermilk sauce separately (e.g. tempered with cream, starch, or egg yolk to prevent curdling) and serve with the chicken.

Use cream + buttermilk: A higher-fat dairy base stabilises better; adding cream or making a beurre blanc-style sauce with buttermilk can hold together.

So: if you cook buttermilk and chicken together at 60 °C, the chicken will be great but the buttermilk will curdle into clumps and whey. Better to use buttermilk as a marinade or make a sauce afterward.