[Fizzlemizz]

These days it would be done using a Mixin eg.
XML

<Button name="AchievementFullSearchResultsButtonTemplate" mixin="AchievementFullSearchResultsButtonMixin" virtual="true">

Lua

AchievementFullSearchResultsButtonMixin = {};
function AchievementFullSearchResultsButtonMixin:Init(elementData)
    -- ...
end

This would "apply" any function defined in the AchievementFullSearchResultsButtonMixin table as a method of the button when the frame using the template is created.