Heat oven to 350°F. Grease 12x8-inch (2-quart) glass baking dish with shortening. In large saucepan, heat chocolate chips and whipping cream over medium-low heat, stirring occasionally, until chips are melted. Stir in 1/3 cup of the brown sugar. Beat in 1 egg yolk at a time until well blended. Continue cooking until slightly thickened. Add butter and vanilla; stir until smooth. Remove from heat; stir in bread cubes.