Cake flour Vs. All purpose flour?

Cake flour has less gluten than all-purpose flour, which can mean getting a tenderer cake. More gluten can make things more chewy; desirable in bread, not so much in cakes. Bread flour has even more gluten than all-purpose. However, you should use whatever your recipe calls for, because using something else might not get the result that was intended. What you are using, with the baking powder included, is called self-raising flour. It also usually has salt added. Don' t use that unless your recipe calls for it or unless you substitute correctly, because it will throw off the proportions of the ingredients. You can look up appropriate substitutions between the different types of flour on cooking ingredient conversion sites.

Using too much leavening(baking powder) can cause a cake to sink. If you are using self-raising flour in a recipe that doesn't call for it, and then also using the baking powder called for, you'd have too much baking powder in the cake.

There are other possible causes, too. Google "causes of cakes sinking" and read a few of the websites you find. I'm sure the answer is there.

Cake flour has less gluten (for lighter texture). You can make it by substituting 2 tbsp AP flour per cup for 2 tbsp of cornstarch & sifting it five times. The cake sinking sounds like an oven temp issue (not hot enough).

Have you ever found any "harina de pastel" or "harina de torta"? I'm pretty sure they have it, somewhere. Maybe a specialty shop.

If you can't find those, you'd be better off using all-purpose flour than making self-raising flour. Sift it three times, and measure it by lightly dumping it into the measuring cup -- also, put in slightly less than the recipe calls for. This should help.