PREHEAT oven to 325º F. Grease baking sheet.
MICROWAVE 2/3 cup chunks in uncovered, microwave-safe bowl on HIGH (100%) power for 45 seconds; STIR. The chunks may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until chunks are melted.
COMBINE flour, baking powder and salt in small bowl. Beat sugar and butter in large bowl until light and fluffy. Add eggs and almond extract; beat until combined. Stir in melted chocolate. Gradually stir in flour mixture. Stir in nuts and 1/2 cup chunks.
SHAPE dough with floured hands on prepared baking sheet into 12 x 3-inch log; round top.
BAKE for 40 minutes or until slightly firm to the touch and cracked on top. Cool on baking sheet for 10 minutes. Slide log onto cutting board and cut into 18 even slices. Return to baking sheet cut-side-down. Bake for an additional 10 to 15 minutes or until firm. Remove to wire rack to cool completely.
PLACE remaining chunks in small, heavy-duty plastic bag. Microwave on HIGH (100%) power for 30 to 45 seconds; knead. Cut a tiny corner from bag and drizzle over cooled biscotti. Let stand until drizzle is set. Store in airtight container at room temperature.