In the Wikipedia article about the Breton delicacy Kouign-amann, there is the following statement:
The Welsh equivalent is the etymologically identical cacan menyn, literally 'cake (of) butter'.
It isn't clear from the article whether there actually is an equivalent Welsh recipe, or if the text is pointing out a similarity between the Welsh and Breton languages.
Is there a Welsh recipe called cacan menyn?
There doesn't appear to be (I know very little Welsh but enough to guess at a few terms with the aid of a dictionary).
There are a few recipes called "cacen menyn" (note the typo in Wikipedia) but they differ from kouign amann, and certainly aren't the celebrated dish that kouign amann is. Various recipes under similar names have less butter and don't use layer in butter and sugar like in kouign amann.
This one for example uses baking soda (the kuoign amann recipes I've seen use yeast) and has added nuts and chocolate icing.
If you can get hold of a copy of "Favourite Welsh teatime recipes" you may find something under the same name.
I've had a whole lot of Welsh desserts based largely on butter :-), but never anything approaching a kouign amann. Never anything approaching it anywhere else than in Brittany in fact, or made by Bretons. Your problem may be a lot greater than a linguistic translation. There's a reason there's not an equivalent in French for instance.