I've made a cake like that - actually, I took the design from another one that appeared here. What I did was to put two circles of a contrasting color on the top of the cake with a nice powdering of cornstarch in between. Then I covered the cake in the color fondant I wanted, making sure this layer of fondant stuck to the top contrasting circle (I think I used a small amount of CMC glue. Once you've done all this, you have three layers of fondant on the top of the cake. The top layer and the middle layer stick together, but the middle layer and the bottom one don't because of the cornstarch. I used an Xacto knife to cut (very carefully!) through the top two layers like I was drawing an asterisk. Then I (again, very carefully!) peeled each piece back and propped then into the position I wanted them to stay in with crumpled kleenex. I let them dry like that for a day or so, the went on decorating the cake. What you end up with is a contrasting color on the inside of the "popped" part, both underneath and also lining the peeled back portions. Here's the link to my cake, along with a big thank you to frostedmemories for the original design:
Now that I'm looking at it again, I probably could have gotten the little triangular pieces to stand up a little more! Oh, well.