Oh, your cake sounds delicious and I’m happy you could a work-a-round and like the frosting. I’m sure everyone will enjoy it! I can usually find the larger malted milk balls at specialty stores or Costco 🙂

I’m sorry you had such trouble with the recipe. When a cake rises a lot and sinks as it cools, that cause is over mixing. My pans are 6″ and the cakes baked right to the top (which is 2″ high). I wish I had more info to give you, but there are so many variables when it comes to baking cakes. Oven temperatures, and being sure the oven holds the temperature (I use an extra thermometer in my oven to ensure the oven is running the proper temperature and preheated for at least 20 minutes, I don’t rely on it just coming to temperature as when you open the oven door a lot of heat escapes, the pans themselves (I use aluminum cake pans, not dark coated pans). I double-checked the recipe and wanted to add that I use large eggs and if by chance extra-large eggs are there could be a problem with the cake rising more than mine did. I’ve made different flavor cakes using this recipe as well as cupcakes without issue. I hope some of these tips help, let me know if you have any more questions.

Always fill cake pans 2/3 full, for room for the cakes to rise properly. If you have extra batter you can make a a cupcake or two out of it, do not over fill cake pans, if cake pans are over fill they will not bake evenly as the center will take at least 15 minutes longer to bake which is why maybe you felt the oven temperature was too low in fact it was just the cake pan being too full abs the cake cannot cake evenly. In filling the three 6” pans I did not have extra batter but hear people who used 8” pans has a little extra batter.

My pans are 6×2. I’m not sure what went wrong. Over mixing or over filling the pans will have them over flow and not bake evenly. There are a lot of variables as to what exactly could have gone wrong. My cakes baked right up to the rim on each cake and not over the rim (no need to trim) I wish I could offer you more support but without any other details that would be difficult. I bake lots of cakes and can say the only time I ever had them over flow was when I over filled the pans-there always has to be enough room for the cake to rise inside the rim. Since I wasn’t in your kitchen when you mixed the cake that is the other variable. If the cake is over mixed the batter will often rise quickly and then fall when it’s reaching the proper cooking time. I will mention if ever I have extra cake batter after I know the pans have their appropriate amount of batter, I don’t over fill them. Using that size pan may have been different for you and maybe you over filled them. Here is the chart I use https://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm