I think that
generating a quarter of a cylinder the way you want it is way easier than cutting it off something else. Try this:
1 - draw a cross-section of half-a-cake, making sure the "central" vertices are on the Y axis.
2 - rotate it by 90° about the Y axis
In order to make it easier to texturize you may now want to combine all top (and bottom) co-planar surfaces - most applications won't frown at that as long as the resulting surface is convex.
Enjoy your cake :-)