Preheat oven to 350 degrees F.
Butter and flour a 9 ½-inch spring form baking pan.
Combine the butter, sugar, and vanilla in a bowl. Cream
together until the mixture is light and fluffy. Beat the eggs in thoroughly one at a time.
In a separate bowl, sift together the flour, baking powder, and salt. Combine with the first mixture and milk. Pour batter into the prepared pan. Bake in the middle of the oven for 50 to 60 minutes, or until a toothpick comes out clean. Remove from oven and let the cake cool in the pan on a rack.
Combine the cornstarch, sugar, milk, eggs, cream, vanilla extract, and salt in a saucepan. Whisk until smooth.
Bring the custard to a boil over moderate heat, whisking constantly. Continuing to whisk, let the custard boil for two minutes.
Remove from heat, and whisk in the butter. Set custard aside to cool, continuing to whisk occasionally.
In a double boiler, melt together the chocolate, water, butter, corn syrup, and salt, stirring until smooth. Remove top of double boiler from heat.
Remove the cake from the pan and cut it in half horizontally with a long serrated knife.
Place one half of the cake on a plate with the cut side facing up. Top with custard.
Place the other half of the cake on top, with the cut side down.
Coat the top of the cake with glaze allowing it to drip down the sides.