The challenge for me was making the perfect filling. It took me four batches to finally get the filling right. That is, at least, my idea of what a Spanish bread filling should be. Sweet— but not overly sweet; buttery with a bit of a texture to it, and it should be moist.
The first batch, I followed what most recipe online uses for a filling: simply mixing butter, sugar, and bread crumbs together and spreading it to the dough before baking. I am not sure if it is the kind of breadcrumbs I used, because they turned out having a dry filling. The other problem is that most of the butter oozed out into the pan while baking. This seems to be a common problem with all the online recipes I found after I read the comments.
The second batch was almost the same, only with different measurements plus it adds powdered milk to it. The taste was good though.
With my third batch, I only used butter and brown sugar. I also smeared some butter on the center of the dough before adding the filling mixture. The outcome…slightly moist filling but there was no texture at all. Just a sticky syrup that also dries out once the Spanish bread gets cold. I recommend this recipe if you are eating them immediately after baking.