I’m afraid it is, ready Sally. And leaf lard in particular. The question is: why? Why won’t butter work just as well? There are a few reasons. The first and most obvious is that butter won’t deliver the same porky flavor, which is all but necessary for a classic pork pie. However there are some functional reasons why you need lard for this particular pork pie.
Leaf lard is a heavily saturated fat — more so than butter — and it’s that saturation that keeps it firm at room temperature. And firmness is an important quality when you need your hand-“raised” pie crust walls to stand up, both while you’re shaping your pie and while it’s baking in the oven.
Can you swap it out for butter in a pinch? Yes you can, though my feeling is you’ll need to bake your pies in forms of some sort if you want to prevent the walls from bowing out severely during the bake: rings, a muffin tin, something like that. Thanks for the question, Sally!