4-1/2 c sugar
½ c Dutch-process cocoa
1-1/2 c cream
½ c unsalted butter, plus more for fingers
3 T light corn syrup
1-1/2 t vanilla

Line an 8-inch baking pan with wax paper.
In a heavy medium saucepan, whisk sugar and cocoa. Add cream, butter and corn syrup; cook over medium-high heat, stirring, until bubbling around the edges, about 5 minutes.
Continue cooking, without stirring, until mixtures reaches soft-ball stage (238-240 on a candy thermometer,) about 12-15 minutes. Wash down sides of pan with a pastry brush dipped in water.
Remove pan from heat and allow to sit until it has cooled to 160, about 40 minutes.
Pour fudge onto clean surface (marble slab, buttered baking sheet,) and use a rubber spatula to gently empty the pan while holding pan very close to surface to prevent spattering. Spoon vanilla over the fudge and let cool until fudge is almost at room temperature, about 15 minutes.
Using a long offset spatula, begin pulling fudge in a figure-8 motion, moving from bottom right up and over to top left, the pull back down to the right, pushing the fudge back onto itself.
Continue until you see a sudden change in appearance, from glossy to matte (12-15 minutes.) Spread into wax-paper lined baking sheet, flatten with lightly buttered fingertips. Cover with wax paper and let stand at room temperature until set, at least 4 hours or overnight.  Cut into squares before serving.