Preparation

• Combine the cream, confectioners’ sugar, and vanilla in a chilled mixing bowl and whisk by hand or with an electric mixer fitted with the whisk attachment until the cream becomes fluffy and forms a soft peak that folds over when you lift the whisk. Be careful not to overwhip the cream or you risk it curdling when you fold it into the custard.
• Whisk the pastry cream until smooth and then gently whisk in about one-third of the whipped cream to lighten the pastry cream. Scrape the rest of the whipped cream over the mixture and, using the whisk in a folding action, gently blend the two until the mixture is uniform and smooth.
• Cut an éclair shell in half lengthwise with a serrated knife. Use your fingers to pinch out the doughy insides of both halves. Use two soupspoons—one to scoop and the other to push the cream off the spoon—to mound the filling into the entire length of the bottom half of the shell, about 2 to 3 Tbs. of filling per shell. Gently place the top half of the éclair shell on the custard and put the assembled éclair on a wire rack set over a rimmed baking sheet. Repeat with the remaining shells. 
• When all the éclairs are assembled, warm the glaze a little just until it’s warm enough to flow off the side of a spoon in a wide, thick ribbon. (If the ganache is too hot, it will run off the éclairs and puddle below on the tray. Spoon the ganache along the entire length of each éclair top. Put the sheet of glazed éclairs in the refrigerator and chill for at least 30 minutes or up to 3 hours before serving.

Make Ahead Tips

The finished éclairs need to chill for at least 30 minutes, but don’t wait longer than 3 hours to serve them.