Cake Preheat the oven to 350 degrees F. Butter and gf flour a 9-1/2” springform pan. Add to the bowl of an electric mixer the butter, sugar and vanilla and beat until light and fluffy. Beat in eggs 1 at a time, beating after each addition. In a separate bowl, mix together flour, baking powder and salt. Add the flour mixture and the milk in alternate batches until all is blended. Pour the batter into the prepared pan and bake for 50 minutes, or until an inserted toothpick comes out clean. Let the cake cool in the pan on a cooling rack.
Custard In a saucepan, whisk together the cornstarch, sugar, milk, eggs, cream and salt until smooth. Add the vanilla scrapings to the custard mixture. Bring to a boil over medium heat, whisking continuously. Let boil for 2 minutes then remove from heat and whisk in butter. Let cool completely, whisking occasionally.
Glaze Place a metal bowl or the top of a double boiler over a saucepan of about an inch of simmering water. Add the chocolate, water, butter, corn syrup and salt stirring until mixture is smooth and all is blended. Remove bowl from the saucepan.
Remove the cake from the pan and cut it in half horizontally with a long serrated knife. Place the bottom half, cut side up on a platter. Top the bottom half with the cooled custard, spreading it to the edges of the cake. Place the top of the cake, cut side down on top of the custard. Pour the glaze over the cake, letting drip down the sides.