Soak poppy seeds in buttermilk for 6 to 8 hours or overnight.
Preheat oven to 180 degrees C, grease a 23cm ring tin. Mix together cinnamon and 1/2 cup sugar. Sprinkle part of the cinnamon sugar mixture evenly over greased tin. This will form a thin crust over the cake. Set aside the remaining cinnamon sugar mixture.
Cream butter and 1 1/2 cup sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Add vanilla.
Sift flour, salt and bicarb soda together. Add to butter mixture alternately with the poppy seed mixture. Blend well.
Put the batter in the ring tin in layers, sprinkling the reserved cinnamon sugar mixture between each layer. If you spread your batter thin, you should get three layers of cinnamon sugar.
Bake for 50 minutes or until a toothpick inserted comes out clean. Set cake on rack to cool for one hour.