The cheesecake milk shake ($7.29) at Katz's Deli (616 Westheimer, 713-521-3838) is no ordinary milk shake. It is liquid ecstasy. To make it, Katz's mixes a large slice of creamy New York-style cheesecake with two large scoops of vanilla ice cream and ice-cold whole milk. (Skim milk? Are you kidding?) Then they whirl it all together and top it off with -- what else? -- whipped cream. The result is an extra-thick, bone-chilling drink/dessert with large chunks of cheesecake that won't fit through any straw.