Heat oven to 325°F. Grease springform pan, 9x2 1/2 inches, with shortening. In 2-quart heavy saucepan, heat 1 cup of the chocolate chunks and 1/2 cup butter over medium heat, stirring occasionally, until melted. Cool 5 minutes. Stir in flour until smooth. Stir in egg yolks until well blended.