Collapsing, dense pound cake

A traditional french sponge cake is typically done by beating the egg with sugar (over a bain marie if you are doing it by hand). Then mix in your zest/flavorings. Then the flour is carefully folded in. Then the melted butter is folded in. If your eggs are starting to curdle when beating then your heat is way too high.

If you want a cake that is large enough to be a wedding cake, you might consider changing the cake type. I think you are pushing your luck with a pound cake in a round pan. If you change the cake type to a sponge, you can still flavor with the lemon zest and cordial (the cordial perhaps is better added after the cake is baked via syrup).

So there are a lot of things that can make a cake collapse, and from reading the comments, I see you've done the basic research. Unfortunately, the best I think anyone can do is make suggestions for how to determine whats wrong.

**Make sure you are weighing your ingredients. Your recipe is in grams, so I assume you are, but it's worth saying.

My first step would be to eliminate the self-rising flour since it is hit-and-miss. You can easily add leavening and salt to regular flour and thus know exactly how much of each you're using, and that eliminates variables.

Since you're taking the time to properly emulsify the butter and eggs, I'd even make a small batch without any chemical leavening and see how that turns out. Too much leavening has the same effect as incorporating too much air and can cause your cake to fall. Chemical leaveners are a fairly recent invention, and technically aren't required for a lot of traditional cakes.

Also, when doing the initial beating, you really want to spend the most time beating the sugar into the butter, and then beat the eggs until just combined. Overbeating the eggs doesn't just risk incorporating too much air and inducing collapse, but can cause your cake to feel dry or crumbly even when it seems a-okay on the outside. I know your recipe says otherwise, but unless you're relying on the egg/butter emulsion for leavening, it seems like an unnecessary risk.

The other thing I notice in your picture is that the outer edge obviously cooked first, and my first impression is that those cakes are rather tall. I've found in the past that the deeper my cake batter, the more prone it is to failure. You could try baking your cakes across more tins. That will help the center set faster compared to what you have now. The edges will still set more quickly, but the difference might not be so great that the whole thing collapses. My mom is a fan or torting her cakes, but I personally find it easier to just bake separate layers for a shorter amount of time.

You could also try reducing your oven's temperature a bit. All oven temperatures fluctuate, and sometimes the oven thermostat will go out of whack (I had a hard time with my landlord for over a year because of a malfunctioning thermostat...). I found a blog post that talks about the changes temperature can bring in a cake: to summarize, cake baked closer to 300 is lighter and fluffier, while cake baked closer to 400 is denser and has more caramelization. After 45 minutes, any cake would be caramelized, so lowering the temperature might be worth a shot. Check out the pictures: https://thecakeblog.com/2015/04/baking-temperature-comparison.html

Those would be my initial tests, and I wouldn't do more than a half-batch of anything while you're experimenting. I'm sure butter is expensive where you live too. And if you really want to be careful, remember: it's best to only change ONE variable at a time. I know it's frustrating, but if you're really at a loss, it's better to make small batches and small changes.

Other things I might try if I can't get the results through ANY of those ideas would include those edge-insulating strips (I've heard you can fold wet paper towels in foil and wrap them around your tins to encourage even rising), and even reducing the amount of cordial by microwaving it or something in case that amount of liquid is messing up the pound cake chemistry.

On a different note: I don't know how much time there is before the wedding, but you could try making a different cake or two and soak it with an elderflower/lemon syrup to get that same kind of flavor. It can be mind-numbing to keep trying the same recipe over and over, especially if something's not going right. If you need a break, or need to prove that you can bake a cake, d*****, try a yellow cake or something, make a nice syrup and ask your friends to try it for comparison. As @aris said, pound cake isn't necessarily the best for a wedding cake. You don't have to think of it as a failure, but let them sample the pound cake and another option or two if you can, and let them decide. Is that particular cake everything they remember, or was it the elderflower cordial that they really loved?

Hope things turn out!

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.

A pound/butter cake is not usually cooked in a big round pan but rather a bread loaf shaped pan for this very reason. I know you wanted to make a buttery rich cake for your friend but this batter will not cook properly because of its density.