i often put buttercream in the centre of my cakes, but the following day after the cake has been covered in fondant i find that there is a bulge where the buttercream is. the last cake i did i made the buttercream thicker but it still hapened, it's as if the cake sinks on itself. do i need to add something to the fondant to make it firmer to hold it's shape better?

Are you using a stiff butter cream dam around the perimeter of the cake before putting the filling in?

Once you do that, place the top layer on and apply slight pressure. This will cause the dam to spread and fill in the gap between the 2 layers.

At this point you'll want to smooth that buttercream dam and apply your crumb coat. I usually allow my cakes to "rest" for a bit and if I'm in a hurry I'll wrap a book with plastic wrap and place it on top of the cake.

This allows the cake to settle and any bulges that occur can easily be smoothed out before applying your final coat of icing or fondant.

Cakes need to settle prior to covering in icing or fondant. Unweighted, settling can take as long as 24 hours, but 12 is usually adequate. I weight cakes with one ceramic tile about the same size as the cake. That way the settling time can be shortened to 2-3 hours.

thanks guys for your feedback. i have been putting the buttercream all the way to the edge of the cake and then putting the top part of the cake back on. i guess any pressure on top is pressing it out. is it best to place then weighed cake in the fridge or jsut leave it out at room temp to set? again thanks, it's not until someone points something out to you that it all makes sense.

I leave mine out at room temp.