Baking – Ascorbic Acid, Calcium Propionate – Differences

Most sources recommend 0.1% or so, but not more than than 0.5%. Try to use only enough to achieve the preservative results you desire.

In other words, for 1000g of flour, start with 1g of calcium propionate. If that works good for you, then stop there. If you still think it's spoiling too soon, then next time try 2g of calcium propionate for 1000g of flour.

Adding more calcium propionate may or may mot make it last any longer, so don't use more than needed — and never more than 5g for 1000 of flour.

Traditional recipes don't seem to aim for goo, and most of the ones you can buy aren't gooey (UK, limited sample of sources). You can try increasing the quantity of spiced, sugared butter (or even mixing a little milk and/or egg in with it) but you may be better off adapting that aspect of another recipe that is supposed to be gooey. You ought to be able to find a cinnamon roll recipe (for example) that states it has such a filling, but swap the cinnamon in the filling for mixed spice. I've been thinking more about this, and about pains au raisin, which tend to have your goo in them. According to Wikipedia they use crème pâtissière.

From my limited experience making Chelsea buns, adjusting the cooking time/temperature won't help - you still want even the middle ones to be cooked through.