My filling for the Brandy Alexander PIe is far too thin. Does this recipe work with Elmlea? I would suggest not!
Nigella's My Brandy Alexander Pie has a mousse-like filling that is made with a combination of melted marshmallows and whipped cream. Nigella uses double cream but the filling should also work with whipping or heavy cream.
Elmlea is a cream substitute that is a combination of buttermilk and vegetable oil. Although you can whip Elmlea, in our experience it takes a very long time to reach soft peaks and far longer than double or whipping cream. So we suspect that it is not really a good subtitute for the pie filling, though you could try to whip the cream subtitute to a slightly firmer texture using an electric hand whisk (being careful not to overwhip) before whisking in the other ingredients. If the marshmallow mixture is too warm this will also soften the whipped cream (either real or a substitute) and turn it soupy, so make sure that the marshmallows have really cooled to room temperature before adding them to the cream.