Summarize:

Egg yolks hold the bit of fat and protein that gives the dish it's rich, filling flavor. If you're feeling extra decadent, go nuts with a bit more yolk and a little less white. Most commonly, you'd add a teaspoon or two of vanilla extract, though there are other options. Add any of the following along with the flour, after most of it is incorporated: Vanilla extract Almond Mint or peppermint Hazelnut Fondants go wonderfully with a fruity or cocoa liquor -- any flavor that would go well with chocolate. Simply fold in 2 tablespoons of your favorite liquor after the flour goes in, then bake like normal. Some ideas include: Chocolate, current, black raspberry, or cherry liquors Coffee spirits or liquors Tia Maria, Cointreau, and other specialty liquors. While most people need some dark chocolate to offset the sweetness of the sugar, you can mix and match milk, dark, and even white chocolate to find your favorite fondant. Simply add whatever chocolate you desire in place of the dark, and melt like normal. Want to hide some "crunch" in your fondants? Add some chocolate chips, of any type, to the cooled batter right before it goes in the oven. A fun-sized candy bar, though decadent, works well too. If you want a little more than chocolate to ooze out of your fondant, you have a few excellent choices. To make them, simply freeze the liquids like ice cubes, then plop the cube into the center of your batter by pouring half in the dish, adding the cube, then covering with batter. Try out: Frozen peanut butter Frozen caramel sauce Frozen jelly (such as raspberry, which pairs well with the dark chocolate_ Frozen chocolate sauce (like a white chocolate center)
Add more egg yolks, and less full eggs, to get a richer cake. Whip some extracts into the batter to give it a punch of flavor. Try a splash of liquor for bit of bite and flavor depth. Experiment with other forms of chocolate. Add an extra "core" to your fondant to increase the depth inside the dish. Finished.