Preheat oven to 180C. Grease and line a lamington pan with baking paper.
Use an electric mixer to beat butter, brown sugar and vanilla, until pale and fluffy. Add the egg and beat until well combined.
Combine buttermilk and egg into butter mix. Add dry ingredients and mix until all combined and mixture is smooth. Pour mixture into prepared pan and smooth the surface. Arrange thinly sliced peach on top and sprinkle with raw sugar and cinnamon.
Bake for 40 minutes or until a skewer inserted into the centre comes out clean. Set aside in the pan for 5 minutes to cool. Cut into pieces and dust top lightly with icing sugar.