Pate a Choux: Preheat oven to 425 degrees F.
Bring salt, water, and butter to a boil. Add flour all at once and stir until it forms a film on the bottom of the pot. Cool mixture and add eggs 1 at a time until completely incorporated. Pipe 5 to 6 inch rods of the mixture onto a lightly greased sheet pan. Bake for 17 minutes. Turn oven down to 350 degrees for 15 more minutes.
Pastry Cream: Scald milk with 3/4-cup sugar and salt. Whisk yolks and remaining 3/4-cup sugar until pale and ribboning. Whisk in flour and cornstarch. Slowly, while whisking, pour in scalded milk. Return all to stove and stir with wooden spoon until thick and 1 or 2 bubbles form. Put mixture in a bowl and add vanilla. Cover with plastic wrap and refrigerate until set, about 1 hour.
Ganache: Melt chocolate and butter together over low heat, stirring constantly.
Using a paring knife, poke a hole into 1 end of each eclair shell. Put a small open tip on a pastry bag and fill bag with cream. Squirt cream into each shell until eclairs feel heavy. Dip top of each filled eclair into chocolate ganache. Serve cold or at room temperature.