I’ve been trying a UK recipe that worked for me last year (I live in Canada). It’s a simple:  220g butter 220g caster sugar 4 eggs (220g weight in shells) 220g self raising flour  lemon zest 3-4 tbsp elderflower cordial   For 6” tins at 350F for 40-45 mins in middle of oven. I’ve also tried to scale it up to 9” tins. The issue I’m having is, 1) the creamed butter and sugar either curdles or is on the verge of curdling with the last egg addition, despite beating the batter really well between additions; 2) the cakes deflate in the middle to form craters during cooking (so before I even take them out of the oven); 3) after removing the outside rings to level the cakes, they are obviously dense, but the flavour is great. I’ve checked my oven temp with an oven thermometer, I’ve tried adding a little less egg, I’ve tried beating the butter and sugar for longer, only just incorporating the flour to avoid over mixing, adding a little extra baking powder and cooking for less time. STILL the cakes sink in the middle and come out dense (I get that this type of cake is more dense anyway, but I can’t stop the middle sinking)! Any ideas or advice would be amazing! I’m trying to make a friends wedding cake with their fave recipe that I made last year! But it’s stressing me out that I can’t seem to fix this :( Thanks so much.  
I used to have a rich fruit cake sink in the middle like this.. I reduced from 350 to 325 and baked a little more time to fix the issue. Sinking is almost always due to cake rising too much too fast and then structure not being able to support it. 