Pour boiling water over the poppy seeds and drain well over a fine sieve. If necessary, repeat this washing. Spread the poppy seeds on a tea towel or newspaper and let dry overnight.
Preheat oven to 180 degrees C. Grease two 22cm cake tins and line bottoms with baking paper. Sift together the flour, baking powder, bicarb soda and 1/4 teaspoon salt. Set aside.
In a large bowl, beat the eggs and 1 1/2 cups sugar until thick and lemon coloured. Beat in the sour cream and vanilla. Sift the flour mixture and gently fold it into the egg mixture, along with the poppy seeds. Pour batter into prepared tins.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Let cool in tin for 10 minutes, then turn out onto a wire rack and cool completely.
In a saucepan, mix together 1/2 cup sugar, cornflour and pinch of salt. Stir in lemon zest, lemon juice, water, orange juice and butter. Cook, stirring constantly, until mixture thickens. Place egg yolks in a medium bowl. Slowly whisk 1/2 cup of hot mixture into egg yolks. Slowly pour the egg yolk mixture back to remaining hot mixture, whisking constantly. Cook, stirring constantly, for 5 minutes or until thickened.