Some friends asked me to make a baby shower cake for a mutual friend--she's struggled with infertility for years, and now she and her husband are finally going to be adopting a little boy who will be born in about three weeks. So it's very meaningful cake, and I wanted to make her something SO special! I think my supercute design would have worked if the top tier hadn't COMPLETELY IMPLODED, more or less. The more I decorated the more it just sunk into a giant pile of bubbling ooze. It's hideous. Pretty sure the problem was the marshmallow fluff I used between the layers. It seems pretty stiff in the jars, but I guess it's just not up to the task without a buttercream dam. Oh well, it may be ugly as sin, but it's going to taste freaking delicious. I just wanted so badly to do something worthy of the unspeakable joy of long-awaited motherhood 
Yup, lesson learned. Thanks for the sympathy, everyone. Now I'm trying to scheme how I could pull off this smores cake differently, with a marshmallow filling that would work.
Hi there- I'm so sorry your cake isn't playing fair!
However, as someone who HAS been through fertility issues, I'm the sure the recipient of the cake will a) just be so thrilled at their bundle being on the way that the issue with the cake won't dampen their spirits in the slightest, and b) they will most likely be very touched that you were so keen to make them a really special cake- it's the thought that counts, so with this in mind and the tips the previous posters have given you, that cake will be very well received I'm sure 
This may sound dumb to every other decorator out there... but I have to do what works for me, so here goes:
After getting tired of having buldges in my cakes even after using a good dam, I decided to sink in the filling layer. So on some of my cakes I take a knife and cut a thin 1/4" or slightly more center out of the top of the cake. I start about 3/4" in from the edge of the cake and follow the shape all the way around with a shallow 1/4" cut, then I hold the knife at an angle and cut a nice smooth area out for the filling to lay. I still add a dam on top of the higher cake edge, but I have no more buldges because the cake itself is helping to hold the filling.
Maybe you could try that with your marshmallow filling since it lays flat and heavy like sadsmile said.
I think it's important to add that I don't torte... I fill between two 2" cakes. So taking 1/4" from the top of one doesn't thin it out.
Go easy on me... I'm just a hobby baker that bakes for family! 
I'm sorry this happened. I am sure they will love their cake no matter what because it was bake with a lot heart!
When I need a marshmallow filling I use Sugarshack's buttercream recipe and add a small tub of Fluff to it. It tastes like vanilla marshmallows and it is really stable.
This may sound dumb to every other decorator out there... but I have to do what works for me, so here goes:
After getting tired of having buldges in my cakes even after using a good dam, I decided to sink in the filling layer. So on some of my cakes I take a knife and cut a thin 1/4" or slightly more center out of the top of the cake. I start about 3/4" in from the edge of the cake and follow the shape all the way around with a shallow 1/4" cut, then I hold the knife at an angle and cut a nice smooth area out for the filling to lay. I still add a dam on top of the higher cake edge, but I have no more buldges because the cake itself is helping to hold the filling.
Maybe you could try that with your marshmallow filling since it lays flat and heavy like sadsmile said.
I think it's important to add that I don't torte... I fill between two 2" cakes. So taking 1/4" from the top of one doesn't thin it out.
Go easy on me... I'm just a hobby baker that bakes for family!
I am so not understanding this...but would like to. And hey...don't worry about criticism...whatever works for you is great and may help others!
And OP...after just having suffered a total cake collapse for a WEDDING, I feel your pain...but know that your friends will not judge you for your cake's appearance, but in the thoughtfulness of your gift. Oh and plan on doing their baby's 1st birthday cake...sans marshmallow filling! 
I understand completely and it sounds like a great idea.
What the poster is saying is that she removes about 1/4" of cake from the bottom layer leaving a "rim" to hold in the filling.
Picture a bowl, that's what she's creating. Genius.
I am so not understanding this...but would like to.
I knew when I was typing it, it was going to be confusing. Sorry.
Imagine it like this... you have a 12" round cake... i set a 10" cardboard cake circle on top of the half that will be filled. I take my carving knife and trace the centered circle, cutting about 1/4" deep. then I shave out the circle section. So I basically hollowed out a circle section of the cake the size and thickness of a cardboard cake circle.
Then i pipe a BC dam around the edge like normal and put my filling in. So the filling is sitting in the cake, as opposed to being stopped by just BC.
I have a cake to make for this weekend, i can take pics to show you if that still doesn't make sense to you.
It does make sense, but I would like to see the pictures anyway if you don't mind. Thanks!!
Thinking of your collapsed cake could you stick a rattle or spoon made of candy clay, chocolate or paste in it and claim it is to represent a smash cake?
Probably a daft idea but...
For myself I will definitely be filing away the method of hollowing a shallow dish into the cake for squishy fillings.
Yours aye
Puss
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