In a large heavy-bottomed sauce pan, combine the sugar, corn syrup, cream, butter and pecans. Stir over medium-high heat until the sugar dissolves. Add the chocolate and stir until melted, then continue to boil slowly until the mixture reaches the firm ball stage, 248 degrees F. (It is best to use a candy thermometer or an instant read thermometer for optimum results.) Remove from the heat and stir in the vanilla. Using a wooden spoon, stir the mixture until it cools slightly and thickens enough to coat the spoon. Working quickly, drop about 1 1/2 tablespoons of the mixture for each praline onto parchment-lined baking sheets. Set aside until the pralines are completely cooled, 15 to 20 minutes.

Store the pralines in a wax paper-lined airtight container.