Bake the eclair shells up to 2 days in advance. If you see that they are beginning to get soft, you can place them back in the oven to crisp. do not place back in the oven once coated or filled. Once filled, they should be kept in the refrigerator until eaten. 

Allow the pastry shells to cool completely, then dip in the ganache to cover the tops. Place in the refrigerator to allow the chocolate to harden.  When ready to fill, spoon the pastry cream into a pastry bag fitted with a "filling" tip, push the tip into the short end of the shell and slowly squeeze the filling in. You will see the pastry plump up a bit, but be careful to to over-fill as the cream will push out through the sides and make a mess if you do.

Pate A Choux
1 cup water
4 oz unsalted butter cut into pieces
1 cup flour
1 tsp sugar
1 tsp salt
3 extra large eggs (about)

Sift sugar, salt and flour together. Place water and butter into a saucepan Heat and bring to a simmer, you want the butter to incorporate into the water, not just float on top.  Remove from heat and add flour mixture all at once. Stir vigorously with a wooden spoon, until it is all mixed together. Place back on heat and continue to stir until the mixture begins to come from the sides of the pan.  You do not want to dry out too much,  but you want to reduce the amount of moisture a bit. Stir over heat about a minute or two.  Place mixture into the bowl of an electric mixer fitted with paddle attachment.  Turn mixer to medium speed to assist bringing the temperature down a bit, but not fully cooled.  Add eggs one at a time, mixing thoroughly after each addition. After adding the last egg check for consistency.  You want the mixture to be a bit firm, and sort of webby. When I say webby I mean it has a consistency where is stretches between the parts sticking to the sides of the bowl and what is on the paddle.  3 eggs should be enough. 
Place in pastry bag and pipe in 6" rods approximately 1" wide,  onto a lined pastry sheet. You can use this same batter for cream puffs, or deep fry for delicious donuts. The batter can be refrigerated for a max of three days before using.  

Vanilla Pastry Cream
Bake in a preheated 425 oven for 15 minutes, then reduce the heat to 375 until the eclairs are browned and crispy.  The interior needs to be dry.  Let cool 

6 large egg yolks

5 Tbs cornstarch

2 1/2 cups milk ( i used 2%)

1/2 cup sugar

1 tsp vanilla

1/4 tsp salt

4 Tbs unsalted butter

for pastry cream:  Whisk the egg yolks and the cornstarch in a bowl. set aside.  In a saucepan, simmer milk, sugar and salt.  

Once it is simmering, temper the eggs with the hot milk.  Return to the saucepan and whisk continuously until the custard begins to thicken. 

When you see large bubbles coming up through the middle of the pot, remove from heat. Pour into a clean bowl

(use a sieve if you feel you have overcooked), and stir in butter and vanilla.  Place clingfilm over top and refrigerate until ready to use. 

Allow to cool completely before filling the eclair shells.

Chocolate Ganache

1/2 lb bittersweet chocolate--chopped

1/2 cup cream

1 Tbs granulated sugar

1 Tbs corn syrup

2 Tbs butter

Heat cream with sugar and corn syrup over medium heat.  Once cream begins to simmer and sugar is dissolved, pour over chocolate and butter.

Let chocolate melt and then whisk together until it is thick and shiny.