Interesting question, Salsmachev! Just for clarification, here are the literal translations:
- Tost a menyn = Toast and butter
- Tost â menyn = Toast with butter
- Tost menyn = Butter-toast
No. 3 is, in fact, what you're most likely to hear for the English expression "buttered toast". As you imply, nos. 1 and 2 (which sound the same, by the way!) refer -- strictly speaking -- to toast and/with butter as separate items.
Supplementary point (hoping not to confuse you!): the parallel expression "bara menyn" (lit. "butter-bread") is used for what English calls "bread and butter" (i.e. buttered bread). This is one of those cases where the Welsh is more consistent than the English!