Traditional petit fours are a layered cake/jam combination - usually three layers of cake spread with a thin, thin layer of jam. In the classic version, there’s a layer of marzipan put on the top, then the cake is cut into squares and coated with poured fondant. The fondant is a little translucent, so you can sometimes see the jam in between the layers, some people don’t care for this so they dip/coat twice. Typically it is a white or yellow cake base and raspberry jam - it would be unusual to do chocolate cake.
What I do is to make my usual layer cake, and torte it very thinly - about a quarter of an inch (I have an Agbay cake leveler just for this), then spread the jam,and assemble the block (usually I do an 11x15 size). I don’t usually use the marzipan, so I cut a board to fit the cake size, and wrap the whole thing in plastic. Then I weight the top to compress it. Then cut into squares and dip into coating chocolate (it covers better, but you have to strain the crumbs out of the chocolate periodically. This is a pretty messy job, because the chocolate hardens in the strainer. I really don’t like making these - the amount of time you spend on them for what you can charge for them doesn’t make it worth my while. There’s a company in CA called Dragonfly Cakes (I think) that is now selling them nationally - one of my distributors is selling this brand - so you might want to take a look there for some inspiration….