With this new Big Mac sauce available from the grocery store, you can replicate the taste of the real McDonald's burger fairly accurately at home.
I built my burger as follows (listing ingredients from the top down):
-top of bun
-sliced pickles
-diced onions
-fried hamburger patty
-Big Mac sauce
-lettuce
-bottom of bun
-diced onions
-fried hamburger patty
-processed cheese slice
-Big Mac sauce
-lettuce
-bottom of bun
The only thing I did wrong was not using sesame seed buns. Regardless, the result was remarkably similar in taste to the real thing.