Put sugar and 2/3 cup water in a heavy medium saucepan. Cook over medium-high heat, stirring occasionally, until sugar has dissolved. Continue to cook, without stirring, until syrup comes to a boil, washing down sides of pan with a wet pastry brush to prevent crystals from forming. Let boil, swirling pan occasionally, until mixture turns medium amber. Remove from heat.
Whisk in butter and cream cheese. Let mixture cool, stirring occasionally. Transfer to an airtight container. Refrigerate overnight; beat until thick enough to hold a ribbon on the surface before using.
These amounts are for a standard two-layer 4-inch-tall tier, with a 1⁄4-inch-thick coating of filling between the layers: 6-inch: 1⁄2 cup to fill; 8-inch: 1 cup to fill; 10-inch: 2 1⁄2 cups to fill; 12-inch: 3 1⁄4 cups to fill. (Note: Many cake makers will split the two cake layers in half horizontally and spread filling between all four resulting layers; if this is your plan, triple the filling amount.)
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