It's getting awfully hard to find mud pie in this town, which is a pity because it's well known that mud pie is the most satisfying dessert ever conceived by humankind. This sentiment is shared by everyone who adores chocolate, and if you don't you must be some kind of a Martian or something. Anyway, mud pie is as good as it is because it combines three items that are already delightful on their own -- mocha ice cream, chocolate cookies, and fudge -- into one almost unspeakably rapturous coalescence. MacArthur Park, a renovated warehouse-turned-rib joint overlooking the waterfront, has been keeping the mud pie flame a-burning for the past three decades. Its rendition is strikingly vertical: A good three inches of Kahlúa-fueled ice cream rises out of a thin yet luscious crust of crushed cookies and pure, unadulterated butter. The pie's crowning glory is the dark, dense, endorphin-rich fudge that runs down its sides and mingles with all that melting ice cream. A big handful of chopped walnuts adds crunchy character to this dreamy, if endangered, dessert.