The Maharal (Gevuros Hashem, ch. 48) proves that matzah ashirah is considered matzah from the fact that the korban minchah is called matzah even though it is made with oil. But in the continuation of ...
Pesachim 39B quotes a Beraisa which says, "The following do not become Chametz: something (i.e. grain) that was already baked (matzo), cooked, or Chalut (scalded, I guess?) in boiling water." Then ...