Cake: Preheat oven to 180 degrees C. Grease and flour one 23cm round cake tin.
Beat the flour, sugar, baking powder, salt, milk, butter, egg and vanilla at low speed scraping bowl constantly for 30 seconds. Beat on high speed scraping bowl occasionally for 3 minutes. Pour batter into the prepared cake tin.
Bake at 180 degrees C for 30 to 35 minutes or until a wooden pick inserted near the centre comes out clean. Remove from the tin and let cool on a wire rack.
Cream Filling: In a 2 litre saucepan mix the sugar, cornflour and salt. Stir in the milk gradually and cook over medium heat stirring constantly until the mixture thickens and boils. Boil and stir 1 minute.
Stir at least 1/2 of the mixture slowly into the egg yolks. Return egg yolk mixture to the saucepan then boil and stir for 1 minute. Remove from heat and stir in the 2 teaspoons vanilla. Let cool to room temperature.
Chocolate Glaze: Heat the chocolate and butter over low heat until melted. Remove from the heat and stir in the icing sugar and vanilla.
Stir in the water one teaspoon at a time until glaze is of desired consistency.
To Assemble the Cake: Split the cooled cake in half to make 2 thin layers. Fill the layers with the filling. Spread the chocolate glaze over the top.