Guess which restaurant icon isn't sweating the economic downturn? One Ronald McDonald, Esq. (pictured here with a fan). McDonald's reported a 5.4% sales increase nationwide last month (compared to last January). The big business is largely attributed to the impeccable quality of the chain's cuisine, as well as its inviting, romantic ambiance. Also, the economy; diners like Tristan Engram, a 22-year-old OTB employee in from Brooklyn, tell the Post he's been slumming it at Mickie D's because the fancier T.G.I. Friday's is now beyond his reach. Which now means the Wall Street execs will have to jockey for the best tables. But besides the value menu, McDonald's has also aggressively expanded into the fancy latte arena formerly dominated by Starbucks; this week the franchise will try to wring some brand loyalty from fashionistas as the official coffee of Mercedes-Benz Fashion Week, serving "espresso-based beverages" at the "exclusive McCafé lounge" in the tents.