Preheat the oven to 375°F. Line two 1/2 sheet pans with parchment paper or silpat. If using parchment paper, put a dab of batter in each corner to glue the paper to the sheet.
Use a piping bag fitted with a 1/2" star tip to pipe 4" long eclairs. Use a sifter to lightly sprinkle the tops of the eclairs with powdered sugar.
Bake until well puffed and golden brown, about 20 minutes. Remove the eclairs from the oven and turn off the heat.
Use a skewer or paring knife to form a small hole in both ends of each eclair. Return the tray to the still warm (but turned off) oven and allow the eclairs to dry for 30 minutes.
Cool to room temperature before filling (see note).
Salted Caramel Pastry Cream
In a large bowl whisk together the egg, yolks and cornstarch until smooth. Set aside.
Place the granulated sugar into a heavy saucepan over medium heat. When the outside edges of the sugar begin to melt use a wooden spoon to stir the melted sugar toward the center. Continue stirring the sugar until it's completely melted.
Watch carefully. When the sugar becomes amber brown turn off the heat.
Add the milk all at once. Keep your face away from the steam that will rise from the pot. Turn the heat back on to low. Add the salt and stir until the caramel is melted into the milk.
Whisk the hot milk into the egg mixture, then return the custard to the pan. Cook over low heat, stirring constantly, until the mixture thickens and just begins to boil.
Immediately strain the pastry cream back into the bowl. Add the butter and vanilla. Stir until the butter is melted. Chill the pastry cream until cold.
Whip the cream with the powdered sugar. Fold the whipped cream into the pastry cream in two batches. Scoop the pastry cream into a pastry bag fitted with a small, plain tip. Pipe the filling into the eclairs from both ends to make sure each eclair is completely filled. Refigerate the eclairs while you make the glaze.
Place the granulated sugar, lemon juice and salt into a heavy saucepan over medium heat. When the outside edges of the sugar begin to melt use a wooden spoon to stir the melted sugar toward the center. Continue stirring the sugar until it's completely melted.
Watch carefully. When sugar becomes light brown turn off the heat. It will continue to brown after being removed from the stove.
VERY CAREFULLY, dip the top of each eclair into the hot caramel and allow the excess to drip back into the pot. NOTE: If you're nervous about dipping the eclairs in hot caramel you can skip this step and sprinkle the eclairs with powdered sugar instead.Do not let your fingers touch the caramel!
Chill the eclairs until serving.
The unfilled eclairs can be packed into freezer bags (don't overcrowd or crush) and frozen for up to a month. To use, line the frozen eclairs on a sheet pan and heat in a 350°F oven for 5 minutes until crisp. Cool and fill.