1.Preheat oven to 160ºC. Grease a deep 23cm round cake pan. Line base and side with baking paper.
2.Beat butter and sugar in a small bowl with electric mixer until light and fluffy. Beat in eggs, one at a time. Transfer mixture to large bowl. Stir in combined milk and sour cream, and sifted dry ingredients in two batches.
3.Spread mixture into pan; tap pan on bench to remove any large air pockets. Bake about 50 minutes. Stand cake in pan 5 minutes before turning, top-side up, onto wire rack to cool.
4.To make sour cream ganache, combine sour cream and chocolate in small saucepan; stir over low heat until smooth. Cool about 45 minutes or until spreadable.
5.Split cake in half, sandwich with half the ganache; spread cake top with remaining ganache.