I will give two options for chocolate cake, and the recipes for fillings.
Easy option for chocolate cake: use whatever chocolate mix you like, but sub out coffee (I usually use decaf, for the who may not want or need caffeine) for half the water. The coffee makes a big difference.
Scratch option for cake:
3 cups AP flour
3 cups granulated sugar
1 1/2 cups cocoa powder
1 tbs baking soda
1 1/2 tbs baking powder
4 large eggs
1 1/2 cups buttermilk
1 1/2 cups warm water or coffee (I would use about 1/2 water, 1/2 coffee)
1/2 cup vegetable oil
2 tsp vanilla
Heat oven to 350, grease and flour pan.
Mix flour, sugar, cocoa, baking soda, and baking powder to combine. Add wet ingredients and mix until combined. Pour into pan and bake 30-35 minutes.
I use a pan that is I think 11"x15". If you wanted to do smaller layers, the bake time should be a bit less.