Livre d'Or

For my own reference, these measurements agree with the ones we used last weekend. You reminded me that the quantities are very approximate, if it's something similar to that it will probably work equally well. In the units I used, for a double-portion, those are:

250g butter (ie. one "normal size" pack)
16 fl oz (2 cup) of brown sugar (pressed down, not loose)
16 gl oz (2 cup) flour (normal white flour for baking sweet things, no yeast)
"Splash" of vanilla
Oven at 180c

How would you describe when the sugar and butter are well mixed? A sort of rich gloopy brown paste? I think I would recognise that as being the desired state, but I'm not sure I could describe it to someone else.