1.Preheat oven to 180°C/350°F. Grease 20cm x 30cm (8-inch x 12-inch) rectangular pan; line base and long sides with baking paper then a strip of foil, extending paper 5cm (2 inches) over sides.
2.Spread chocolate evenly over foil in base of pan. Refrigerate about 10 minutes or until set.
3.Meanwhile, beat sour cream, sugar and butter in small bowl with electric mixer until smooth and creamy. Add sifted flour and remaining ingredients. Beat on medium speed about 3 minutes or until mixture is changed to a paler colour; spread over chocolate.
4.Bake about 25 minutes. Cool slice in pan 10 minutes, then refrigerate about 30 minutes or until chocolate sets.
5.To make orange glaze, combine ingredients in small bowl.
6.Turn slice top-side down onto board, carefully remove paper and foil from base. Turn slice top-side up; spread slice with glaze. Refrigerate until set before cutting.
Store squares in an airtight container in the fridge for up to 3 days.Note