Prepare an ice bath by filling a large bowl with ice and cold water, then nestle a medium-sized bowl inside.

In a medium saucepan off of heat, whisk together sugar, cornstarch and salt. Very gradually whisk in milk to dissolve cornstarch, then whisk in egg yolks.

Stirring constantly, cook over medium heat until the first large bubbles form and sputter. Reduce heat to low and, still stirring, cook until cream is thick, one or two minutes more. Remove from heat and pour cream into the medium bowl set in ice bath. (Force it through a fine-mesh sieve if the cream has curdled - no biggie.) Stir butter and vanilla into hot pastry cream, then place bowl into ice bath and stir continuously until cooled.

In a medium saucepan, combine whipping cream, corn syrup, butter and salt over medium high heat. As soon as it comes to a simmer, remove from heat.

Place chopped chocolate in a heat-proof bowl and pour cream mixture over top. Stir continually until chocolate has melted completely.

Use a sharp knife to cut the rounded top off of the bottom cake, carving out a shallow well for the filling.  Fill with the pastry cream, spreading to the edges of cake.  Position second cake on top.  Spread ganache over top (rewarm it first over a saucepan of simmering water if needed).  Refrigerate until ready to serve.