I need some advice. I am going to be making a sheet cake for someone that needs about 30 servings. I was thinking maybe an 11x15? My biggest concern is that they would like chocolate in the middle, and I'm a little afraid that cutting it in half to fill it would cause it to break. My other idea is that maybe it would be easier to do a one layer 10inch square beside a 5 inch (10 inch cut in half), its just that i've never done that and don't know how easy it would to be to "seam" together. Thank You!

9x13 is the perfect size for 30 and if you don't want to cut it in half divide the batter in 2 pans.

A single layer 9x13 only serves 15-20. A single layer 12x18 will serve 30-50. If you want them filled, work with them cold from the frig, or slighly frozen. After splitting, place a cake board, or cookie sheet to remove the top layer, fill, then slide the top layer on. If if splits a little...that's ok...after you ice it, no one will know the difference. It will all seam back together.

I split them in half all the time and fill. I just split and use a cakeboard to slip between the layers. Lift the top off and set aside. Fill. Then gently replace the top. Dont panic if it cracks a little. It can be patched. I do it so often, they rarely crack for me.

I have piped filling into a sheet cake where the individual pieces would be cut, like filling a cupcake and it worked out fine. Maybe that would be an easier alternative.

In case I wasn't clear I was planning on a single layer (2inch) cut in half.